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[p5sagit/p5-mst-13.2.git] / pod / perlfunc.pod

=head1 NAME

perlfunc - Perl builtin functions

=head1 DESCRIPTION

The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators.  These differ in their precedence relationship with a
following comma.  (See the precedence table in L<perlop>.)  List
operators take more than one argument, while unary operators can never
take more than one argument.  Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator.  A unary operator generally provides a scalar context to its
argument, while a list operator may provide either scalar or list
contexts for its arguments.  If it does both, the scalar arguments will
be first, and the list argument will follow.  (Note that there can ever
be only one such list argument.)  For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.

In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
with LIST as an argument.  Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
Elements of the LIST should be separated by commas.

Any function in the list below may be used either with or without
parentheses around its arguments.  (The syntax descriptions omit the
parentheses.)  If you use the parentheses, the simple (but occasionally
surprising) rule is this: It I<looks> like a function, therefore it I<is> a
function, and precedence doesn't matter.  Otherwise it's a list
operator or unary operator, and precedence does matter.  And whitespace
between the function and left parenthesis doesn't count--so you need to
be careful sometimes:

    print 1+2+4;        # Prints 7.
    print(1+2) + 4;     # Prints 3.
    print (1+2)+4;      # Also prints 3!
    print +(1+2)+4;     # Prints 7.
    print ((1+2)+4);    # Prints 7.

If you run Perl with the B<-w> switch it can warn you about this.  For
example, the third line above produces:

    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.

A few functions take no arguments at all, and therefore work as neither
unary nor list operators.  These include such functions as C<time>
and C<endpwent>.  For example, C<time+86_400> always means
C<time() + 86_400>.

For functions that can be used in either a scalar or list context,
nonabortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.

Remember the following important rule: There is B<no rule> that relates
the behavior of an expression in list context to its behavior in scalar
context, or vice versa.  It might do two totally different things.
Each operator and function decides which sort of value it would be most
appropriate to return in scalar context.  Some operators return the
length of the list that would have been returned in list context.  Some
operators return the first value in the list.  Some operators return the
last value in the list.  Some operators return a count of successful
operations.  In general, they do what you want, unless you want
consistency.

An named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context.  You can't get a list
like C<(1,2,3)> into being in scalar context, because the compiler knows
the context at compile time.  It would generate the scalar comma operator
there, not the list construction version of the comma.  That means it
was never a list to start with.

In general, functions in Perl that serve as wrappers for system calls
of the same name (like chown(2), fork(2), closedir(2), etc.) all return
true when they succeed and C<undef> otherwise, as is usually mentioned
in the descriptions below.  This is different from the C interfaces,
which return C<-1> on failure.  Exceptions to this rule are C<wait>,
C<waitpid>, and C<syscall>.  System calls also set the special C<$!>
variable on failure.  Other functions do not, except accidentally.

=head2 Perl Functions by Category

Here are Perl's functions (including things that look like
functions, like some keywords and named operators)
arranged by category.  Some functions appear in more
than one place.

=over

=item Functions for SCALARs or strings

C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>

=item Regular expressions and pattern matching

C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>

=item Numeric functions

C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
C<sin>, C<sqrt>, C<srand>

=item Functions for real @ARRAYs

C<pop>, C<push>, C<shift>, C<splice>, C<unshift>

=item Functions for list data

C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>

=item Functions for real %HASHes

C<delete>, C<each>, C<exists>, C<keys>, C<values>

=item Input and output functions

C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
C<warn>, C<write>

=item Functions for fixed length data or records

C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>

=item Functions for filehandles, files, or directories

C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
C<unlink>, C<utime>

=item Keywords related to the control flow of your perl program

C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>

=item Keywords related to scoping

C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>

=item Miscellaneous functions

C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
C<scalar>, C<undef>, C<wantarray>

=item Functions for processes and process groups

C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
C<times>, C<wait>, C<waitpid>

=item Keywords related to perl modules

C<do>, C<import>, C<no>, C<package>, C<require>, C<use>

=item Keywords related to classes and object-orientedness

C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
C<untie>, C<use>

=item Low-level socket functions

C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
C<socket>, C<socketpair>

=item System V interprocess communication functions

C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>

=item Fetching user and group info

C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
C<getpwuid>, C<setgrent>, C<setpwent>

=item Fetching network info

C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
C<setnetent>, C<setprotoent>, C<setservent>

=item Time-related functions

C<gmtime>, C<localtime>, C<time>, C<times>

=item Functions new in perl5

C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>

* - C<sub> was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.

=item Functions obsoleted in perl5

C<dbmclose>, C<dbmopen>

=back

=head2 Portability

Perl was born in Unix and can therefore access all common Unix
system calls.  In non-Unix environments, the functionality of some
Unix system calls may not be available, or details of the available
functionality may differ slightly.  The Perl functions affected
by this are:

C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
C<utime>, C<wait>, C<waitpid>

For more information about the portability of these functions, see
L<perlport> and other available platform-specific documentation.

=head2 Alphabetical Listing of Perl Functions

=over 8

=item I<-X> FILEHANDLE

=item I<-X> EXPR

=item I<-X>

A file test, where X is one of the letters listed below.  This unary
operator takes one argument, either a filename or a filehandle, and
tests the associated file to see if something is true about it.  If the
argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
Unless otherwise documented, it returns C<1> for true and C<''> for false, or
the undefined value if the file doesn't exist.  Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator.  The
operator may be any of:
X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>

    -r  File is readable by effective uid/gid.
    -w  File is writable by effective uid/gid.
    -x  File is executable by effective uid/gid.
    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.
    -W  File is writable by real uid/gid.
    -X  File is executable by real uid/gid.
    -O  File is owned by real uid.

    -e  File exists.
    -z  File has zero size.
    -s  File has nonzero size (returns size).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO), or Filehandle is a pipe.
    -S  File is a socket.
    -b  File is a block special file.
    -c  File is a character special file.
    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.
    -g  File has setgid bit set.
    -k  File has sticky bit set.

    -T  File is an ASCII text file.
    -B  File is a "binary" file (opposite of -T).

    -M  Age of file in days when script started.
    -A  Same for access time.
    -C  Same for inode change time.

Example:

    while (<>) {
        chop;
        next unless -f $_;      # ignore specials
        #...
    }

The interpretation of the file permission operators C<-r>, C<-R>,
C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
of the file and the uids and gids of the user.  There may be other
reasons you can't actually read, write, or execute the file.  Such
reasons may be for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and unrecognized
executable formats.

Also note that, for the superuser on the local filesystems, the C<-r>,
C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
if any execute bit is set in the mode.  Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file,
or temporarily set their effective uid to something else.

If you are using ACLs, there is a pragma called C<filetest> that may
produce more accurate results than the bare stat() mode bits.
When under the C<use filetest 'access'> the above-mentioned filetests
will test whether the permission can (not) be granted using the
access() family of system calls.  Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs).  This strangeness is
due to the underlying system calls' definitions.  Read the
documentation for the C<filetest> pragma for more information.

Note that C<-s/a/b/> does not do a negated substitution.  Saying
C<-exp($foo)> still works as expected, however--only single letters
following a minus are interpreted as file tests.

The C<-T> and C<-B> switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many strange characters (E<gt>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file.  Also, any file
containing null in the first block is considered a binary file.  If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
rather than the first block.  Both C<-T> and C<-B> return true on a null
file, or a file at EOF when testing a filehandle.  Because you have to
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.

If any of the file tests (or either the C<stat> or C<lstat> operators) are given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call.  (This doesn't work with C<-t>, and you need to remember
that lstat() and C<-l> will leave values in the stat structure for the
symbolic link, not the real file.)  Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;

=item abs VALUE

=item abs

Returns the absolute value of its argument.
If VALUE is omitted, uses C<$_>.

=item accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the accept(2) system call
does.  Returns the packed address if it succeeded, false otherwise.
See the example in L<perlipc/"Sockets: Client/Server Communication">.

=item alarm SECONDS

=item alarm

Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed.  If SECONDS is not specified,
the value stored in C<$_> is used. (On some machines,
unfortunately, the elapsed time may be up to one second less than you
specified because of how seconds are counted.)  Only one timer may be
counting at once.  Each call disables the previous timer, and an
argument of C<0> may be supplied to cancel the previous timer without
starting a new one.  The returned value is the amount of time remaining
on the previous timer.

For delays of finer granularity than one second, you may use Perl's
four-argument version of select() leaving the first three arguments
undefined, or you might be able to use the C<syscall> interface to
access setitimer(2) if your system supports it.  The Time::HiRes module
from CPAN may also prove useful.

It is usually a mistake to intermix C<alarm> and C<sleep> calls.
(C<sleep> may be internally implemented in your system with C<alarm>)

If you want to use C<alarm> to time out a system call you need to use an
C<eval>/C<die> pair.  You can't rely on the alarm causing the system call to
fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
restart system calls on some systems.  Using C<eval>/C<die> always works,
modulo the caveats given in L<perlipc/"Signals">.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    if ($@) {
        die unless $@ eq "alarm\n";   # propagate unexpected errors
        # timed out
    }
    else {
        # didn't
    }

=item atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the C<POSIX::tan()>
function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

=item bind SOCKET,NAME

Binds a network address to a socket, just as the bind system call
does.  Returns true if it succeeded, false otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in "binary" mode on
systems whose run-time libraries force the programmer to guess
between binary and text files.  If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.  binmode() should be
called after the C<open> but before any I/O is done on the filehandle.
The only way to reset binary mode on a filehandle is to reopen the
file.

The operating system, device drivers, C libraries, and Perl run-time
system all conspire to let the programmer conveniently treat a
simple, one-byte C<\n> as the line terminator, irrespective of its
external representation.  On Unix and its brethren, the native file
representation exactly matches the internal representation, making
everyone's lives unbelievably simpler.  Consequently, L<binmode>
has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
to end each line.  (Unix and Plan9 think C<\n> means C<\cJ> and
C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
C<\cM> for c<\n> and C<\cJ> to mean C<\r>.  But that's ok, because
it's only one byte, and the internal and external representations
match.)

In legacy systems like MS-DOS and its embellishments, your program
sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
that's not what's physically stored on disk.  What's worse, these
systems refuse to help you with this; it's up to you to remember
what to do.  And you mustn't go applying binmode() with wild abandon,
either, because if your system does care about binmode(), then using
it when you shouldn't is just as perilous as failing to use it when
you should.

That means that on any version of Microsoft WinXX that you might
care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
to be converted to C<\n> when read into your program, and causes
any C<\n> in your program to be converted back to C<\cM\cJ> on
output to disk.  This sad discrepancy leads to no end of
problems in not just the readline operator, but also when using
seek(), tell(), and read() calls.  See L<perlport> for other painful
details.  See the C<$/> and C<$\> variables in L<perlvar> for how
to manually set your input and output line-termination sequences.

=item bless REF,CLASSNAME

=item bless REF

This function tells the thingy referenced by REF that it is now an object
in the CLASSNAME package.  If CLASSNAME is omitted, the current package
is used.  Because a C<bless> is often the last thing in a constructor,
it returns the reference for convenience.  Always use the two-argument
version if the function doing the blessing might be inherited by a
derived class.  See L<perltoot> and L<perlobj> for more about the blessing
(and blessings) of objects.

Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Perl pragmata.  Builtin types have all uppercase names, so to prevent
confusion, you may wish to avoid such package names as well.  Make sure
that CLASSNAME is a true value.

See L<perlmod/"Perl Modules">.

=item caller EXPR

=item caller

Returns the context of the current subroutine call.  In scalar context,
returns the caller's package name if there is a caller, that is, if
we're in a subroutine or C<eval> or C<require>, and the undefined value
otherwise.  In list context, returns

    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to
print a stack trace.  The value of EXPR indicates how many call frames
to go back before the current one.

    ($package, $filename, $line, $subroutine, $hasargs,
    $wantarray, $evaltext, $is_require, $hints) = caller($i);

Here $subroutine may be C<(eval)> if the frame is not a subroutine
call, but an C<eval>.  In such a case additional elements $evaltext and
C<$is_require> are set: C<$is_require> is true if the frame is created by a
C<require> or C<use> statement, $evaltext contains the text of the
C<eval EXPR> statement.  In particular, for a C<eval BLOCK> statement,
$filename is C<(eval)>, but $evaltext is undefined.  (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
frame.  C<$hints> contains pragmatic hints that the caller was
compiled with.  It currently only reflects the hint corresponding to
C<use utf8>.

Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable C<@DB::args> to be the
arguments with which the subroutine was invoked.

Be aware that the optimizer might have optimized call frames away before
C<caller> had a chance to get the information.  That means that C<caller(N)>
might not return information about the call frame you expect it do, for
C<N E<gt> 1>.  In particular, C<@DB::args> might have information from the 
previous time C<caller> was called.

=item chdir EXPR

Changes the working directory to EXPR, if possible.  If EXPR is omitted,
changes to the user's home directory.  Returns true upon success,
false otherwise.  See the example under C<die>.

=item chmod LIST

Changes the permissions of a list of files.  The first element of the
list must be the numerical mode, which should probably be an octal
number, and which definitely should I<not> a string of octal digits:
C<0644> is okay, C<'0644'> is not.  Returns the number of files
successfully changed.  See also L</oct>, if all you have is a string.

    $cnt = chmod 0755, 'foo', 'bar';
    chmod 0755, @executables;
    $mode = '0644'; chmod $mode, 'foo';      # !!! sets mode to
                                             # --w----r-T
    $mode = '0644'; chmod oct($mode), 'foo'; # this is better
    $mode = 0644;   chmod $mode, 'foo';      # this is best

=item chomp VARIABLE

=item chomp LIST

=item chomp

This safer version of L</chop> removes any trailing string
that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the total
number of characters removed from all its arguments.  It's often used to
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline.  When in paragraph
mode (C<$/ = "">), it removes all trailing newlines from the string.
When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
a reference to an integer or the like, see L<perlvar>) chomp() won't
remove anything.  
If VARIABLE is omitted, it chomps C<$_>.  Example:

    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        # ...
    }

You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of
characters removed is returned.

=item chop VARIABLE

=item chop LIST

=item chop

Chops off the last character of a string and returns the character
chopped.  It's used primarily to remove the newline from the end of an
input record, but is much more efficient than C<s/\n//> because it neither
scans nor copies the string.  If VARIABLE is omitted, chops C<$_>.
Example:

    while (<>) {
        chop;   # avoid \n on last field
        @array = split(/:/);
        #...
    }

You can actually chop anything that's an lvalue, including an assignment:

    chop($cwd = `pwd`);
    chop($answer = <STDIN>);

If you chop a list, each element is chopped.  Only the value of the
last C<chop> is returned.

Note that C<chop> returns the last character.  To return all but the last
character, use C<substr($string, 0, -1)>.

=item chown LIST

Changes the owner (and group) of a list of files.  The first two
elements of the list must be the I<numeric> uid and gid, in that
order.  A value of -1 in either position is interpreted by most
systems to leave that value unchanged.  Returns the number of files
successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

Here's an example that looks up nonnumeric uids in the passwd file:

    print "User: ";
    chomp($user = <STDIN>);
    print "Files: ";
    chomp($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = glob($pattern);      # expand filenames
    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups.  On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.
On POSIX systems, you can detect this condition this way:

    use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
    $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

=item chr NUMBER

=item chr

Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face (but only within the scope of
a C<use utf8>).  For the reverse, use L</ord>.  
See L<utf8> for more about Unicode.

If NUMBER is omitted, uses C<$_>.

=item chroot FILENAME

=item chroot

This function works like the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
begin with a C</> by your process and all its children.  (It doesn't
change your current working directory, which is unaffected.)  For security
reasons, this call is restricted to the superuser.  If FILENAME is
omitted, does a C<chroot> to C<$_>.

=item close FILEHANDLE

=item close

Closes the file or pipe associated with the file handle, returning true
only if stdio successfully flushes buffers and closes the system file
descriptor.  Closes the currently selected filehandle if the argument
is omitted.

You don't have to close FILEHANDLE if you are immediately going to do
another C<open> on it, because C<open> will close it for you.  (See
C<open>.)  However, an explicit C<close> on an input file resets the line
counter (C<$.>), while the implicit close done by C<open> does not.

If the file handle came from a piped open C<close> will additionally
return false if one of the other system calls involved fails or if the
program exits with non-zero status.  (If the only problem was that the
program exited non-zero C<$!> will be set to C<0>.)  Closing a pipe 
also waits for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards, and 
implicitly puts the exit status value of that command into C<$?>.

Prematurely closing the read end of a pipe (i.e. before the process
writing to it at the other end has closed it) will result in a
SIGPIPE being delivered to the writer.  If the other end can't
handle that, be sure to read all the data before closing the pipe.

Example:

    open(OUTPUT, '|sort >foo')  # pipe to sort
        or die "Can't start sort: $!";
    #...                        # print stuff to output
    close OUTPUT                # wait for sort to finish
        or warn $! ? "Error closing sort pipe: $!"
                   : "Exit status $? from sort";
    open(INPUT, 'foo')          # get sort's results
        or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect
filehandle, usually the real filehandle name.

=item closedir DIRHANDLE

Closes a directory opened by C<opendir> and returns the success of that
system call.

DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name.

=item connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call
does.  Returns true if it succeeded, false otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item continue BLOCK

Actually a flow control statement rather than a function.  If there is a
C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the C<next> statement (which is similar to the C C<continue>
statement).

C<last>, C<next>, or C<redo> may appear within a C<continue>
block.  C<last> and C<redo> will behave as if they had been executed within
the main block.  So will C<next>, but since it will execute a C<continue>
block, it may be more entertaining.

    while (EXPR) {
        ### redo always comes here
        do_something;
    } continue {
        ### next always comes here
        do_something_else;
        # then back the top to re-check EXPR
    }
    ### last always comes here

Omitting the C<continue> section is semantically equivalent to using an
empty one, logically enough.  In that case, C<next> goes directly back
to check the condition at the top of the loop.

=item cos EXPR

Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted,
takes cosine of C<$_>.

For the inverse cosine operation, you may use the C<POSIX::acos()>
function, or use this relation:

    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

=item crypt PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) function in the C library
(assuming that you actually have a version there that has not been
extirpated as a potential munition).  This can prove useful for checking
the password file for lousy passwords, amongst other things.  Only the
guys wearing white hats should do this.

Note that C<crypt> is intended to be a one-way function, much like breaking
eggs to make an omelette.  There is no (known) corresponding decrypt
function.  As a result, this function isn't all that useful for
cryptography.  (For that, see your nearby CPAN mirror.)

When verifying an existing encrypted string you should use the encrypted
text as the salt (like C<crypt($plain, $crypted) eq $crypted>).  This
allows your code to work with the standard C<crypt> and with more
exotic implementations.  When choosing a new salt create a random two
character string whose characters come from the set C<[./0-9A-Za-z]>
(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).

Here's an example that makes sure that whoever runs this program knows
their own password:

    $pwd = (getpwuid($<))[1];

    system "stty -echo";
    print "Password: ";
    chomp($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }

Of course, typing in your own password to whoever asks you
for it is unwise.

The L<crypt> function is unsuitable for encrypting large quantities
of data, not least of all because you can't get the information
back.  Look at the F<by-module/Crypt> and F<by-module/PGP> directories
on your favorite CPAN mirror for a slew of potentially useful
modules.

=item dbmclose HASH

[This function has been largely superseded by the C<untie> function.]

Breaks the binding between a DBM file and a hash.

=item dbmopen HASH,DBNAME,MASK

[This function has been largely superseded by the C<tie> function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash.  HASH is the name of the hash.  (Unlike normal C<open>, the first
argument is I<not> a filehandle, even though it looks like one).  DBNAME
is the name of the database (without the F<.dir> or F<.pag> extension if
any).  If the database does not exist, it is created with protection
specified by MASK (as modified by the C<umask>).  If your system supports
only the older DBM functions, you may perform only one C<dbmopen> in your
program.  In older versions of Perl, if your system had neither DBM nor
ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
sdbm(3).

If you don't have write access to the DBM file, you can only read hash
variables, not set them.  If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an C<eval>,
which will trap the error.

Note that functions such as C<keys> and C<values> may return huge lists
when used on large DBM files.  You may prefer to use the C<each>
function to iterate over large DBM files.  Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also L<AnyDBM_File> for a more general description of the pros and
cons of the various dbm approaches, as well as L<DB_File> for a particularly
rich implementation.

You can control which DBM library you use by loading that library
before you call dbmopen():

    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
        or die "Can't open netscape history file: $!";

=item defined EXPR

=item defined

Returns a Boolean value telling whether EXPR has a value other than
the undefined value C<undef>.  If EXPR is not present, C<$_> will be
checked.

Many operations return C<undef> to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish C<undef> from
other values.  (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and C<"0">, which are all equally
false.)  Note that since C<undef> is a valid scalar, its presence
doesn't I<necessarily> indicate an exceptional condition: C<pop>
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.

You may also use C<defined(&func)> to check whether subroutine C<&func>
has ever been defined.  The return value is unaffected by any forward
declarations of C<&foo>.

Use of C<defined> on aggregates (hashes and arrays) is deprecated.  It
used to report whether memory for that aggregate has ever been
allocated.  This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }

When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash.  Use L</exists> for the latter
purpose.

Examples:

    print if defined $switch{'D'};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
        unless defined($value = readlink $sym);
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
    $debugging = 0 unless defined $debugging;

Note:  Many folks tend to overuse C<defined>, and then are surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values.  For example, if you say

    "ab" =~ /a(.*)b/;

The pattern match succeeds, and C<$1> is defined, despite the fact that it
matched "nothing".  But it didn't really match nothing--rather, it
matched something that happened to be zero characters long.  This is all
very above-board and honest.  When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer.  So you
should use C<defined> only when you're questioning the integrity of what
you're trying to do.  At other times, a simple comparison to C<0> or C<""> is
what you want.

See also L</undef>, L</exists>, L</ref>.

=item delete EXPR

Deletes the specified key(s) and their associated values from a hash.
For each key, returns the deleted value associated with that key, or
the undefined value if there was no such key.  Deleting from C<$ENV{}>
modifies the environment.  Deleting from a hash tied to a DBM file
deletes the entry from the DBM file.  (But deleting from a C<tie>d hash
doesn't necessarily return anything.)

The following deletes all the values of a hash:

    foreach $key (keys %HASH) {
        delete $HASH{$key};
    }

And so does this:

    delete @HASH{keys %HASH}

But both of these are slower than just assigning the empty list
or undefining it:

    %hash = ();         # completely empty %hash
    undef %hash;        # forget %hash every existed

Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash element lookup or hash slice:

    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

=item die LIST

Outside an C<eval>, prints the value of LIST to C<STDERR> and
exits with the current value of C<$!> (errno).  If C<$!> is C<0>,
exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
status).  If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>.  Inside
an C<eval(),> the error message is stuffed into C<$@> and the
C<eval> is terminated with the undefined value.  This makes
C<die> the way to raise an exception.

Equivalent examples:

    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
is supplied.  Note that the "input line number" (also known as "chunk")
is subject to whatever notion of "line" happens to be currently in
effect, and is also available as the special variable C<$.>.
See L<perlvar/"$/"> and L<perlvar/"$.">.

Hint: sometimes appending C<", stopped"> to your message
will cause it to make better sense when the string C<"at foo line 123"> is
appended.  Suppose you are running script "canasta".

    die "/etc/games is no good";
    die "/etc/games is no good, stopped";

produce, respectively

    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.

See also exit(), warn(), and the Carp module.

If LIST is empty and C<$@> already contains a value (typically from a
previous eval) that value is reused after appending C<"\t...propagated">.
This is useful for propagating exceptions:

    eval { ... };
    die unless $@ =~ /Expected exception/;

If C<$@> is empty then the string C<"Died"> is used.

die() can also be called with a reference argument.  If this happens to be
trapped within an eval(), $@ contains the reference.  This behavior permits
a more elaborate exception handling implementation using objects that
maintain arbitary state about the nature of the exception.  Such a scheme
is sometimes preferable to matching particular string values of $@ using
regular expressions.  Here's an example:

    eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
    if ($@) {
        if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
            # handle Some::Module::Exception
        }
        else {
            # handle all other possible exceptions
        }
    }

Because perl will stringify uncaught exception messages before displaying
them, you may want to overload stringification operations on such custom
exception objects.  See L<overload> for details about that.

You can arrange for a callback to be run just before the C<die>
does its deed, by setting the C<$SIG{__DIE__}> hook.  The associated
handler will be called with the error text and can change the error
message, if it sees fit, by calling C<die> again.  See
L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
L<"eval BLOCK"> for some examples.  Although this feature was meant
to be run only right before your program was to exit, this is not
currently the case--the C<$SIG{__DIE__}> hook is currently called
even inside eval()ed blocks/strings!  If one wants the hook to do
nothing in such situations, put

        die @_ if $^S;

as the first line of the handler (see L<perlvar/$^S>).  Because
this promotes strange action at a distance, this counterintuitive
behavior may be fixed in a future release.  

=item do BLOCK

Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by a loop
modifier, executes the BLOCK once before testing the loop condition.
(On other statements the loop modifiers test the conditional first.)

C<do BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
See L<perlsyn> for alternative strategies.

=item do SUBROUTINE(LIST)

A deprecated form of subroutine call.  See L<perlsub>.

=item do EXPR

Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.  Its primary use is to include subroutines
from a Perl subroutine library.

    do 'stat.pl';

is just like

    scalar eval `cat stat.pl`;

except that it's more efficient and concise, keeps track of the current
filename for error messages, searches the @INC libraries, and updates
C<%INC> if the file is found.  See L<perlvar/Predefined Names> for these
variables.  It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does.  It's the
same, however, in that it does reparse the file every time you call it,
so you probably don't want to do this inside a loop.

If C<do> cannot read the file, it returns undef and sets C<$!> to the
error.  If C<do> can read the file but cannot compile it, it
returns undef and sets an error message in C<$@>.   If the file is
successfully compiled, C<do> returns the value of the last expression
evaluated.

Note that inclusion of library modules is better done with the
C<use> and C<require> operators, which also do automatic error checking
and raise an exception if there's a problem.

You might like to use C<do> to read in a program configuration
file.  Manual error checking can be done this way:

    # read in config files: system first, then user 
    for $file ("/share/prog/defaults.rc",
               "$ENV{HOME}/.someprogrc") 
   {
        unless ($return = do $file) {
            warn "couldn't parse $file: $@" if $@;
            warn "couldn't do $file: $!"    unless defined $return;
            warn "couldn't run $file"       unless $return;
        }
    }

=item dump LABEL

=item dump

This function causes an immediate core dump.  See also the B<-u>
command-line switch in L<perlrun>, which does the same thing.
Primarily this is so that you can use the B<undump> program (not
supplied) to turn your core dump into an executable binary after
having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing
a C<goto LABEL> (with all the restrictions that C<goto> suffers).
Think of it as a goto with an intervening core dump and reincarnation.
If C<LABEL> is omitted, restarts the program from the top.

B<WARNING>: Any files opened at the time of the dump will I<not>
be open any more when the program is reincarnated, with possible
resulting confusion on the part of Perl.  

This function is now largely obsolete, partly because it's very
hard to convert a core file into an executable, and because the
real compiler backends for generating portable bytecode and compilable
C code have superseded it.

If you're looking to use L<dump> to speed up your program, consider
generating bytecode or native C code as described in L<perlcc>.  If
you're just trying to accelerate a CGI script, consider using the
C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
You might also consider autoloading or selfloading, which at least
make your program I<appear> to run faster.  

=item each HASH

When called in list context, returns a 2-element list consisting of the
key and value for the next element of a hash, so that you can iterate over
it.  When called in scalar context, returns the key for only the "next"
element in the hash.  (Note: Keys may be C<"0"> or C<"">, which are logically
false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
for this reason.)

Entries are returned in an apparently random order.  The actual random
order is subject to change in future versions of perl, but it is guaranteed
to be in the same order as either the C<keys> or C<values> function
would produce on the same (unmodified) hash.

When the hash is entirely read, a null array is returned in list context
(which when assigned produces a false (C<0>) value), and C<undef> in
scalar context.  The next call to C<each> after that will start iterating
again.  There is a single iterator for each hash, shared by all C<each>,
C<keys>, and C<values> function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating C<keys HASH> or
C<values HASH>.  If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so don't.

The following prints out your environment like the printenv(1) program,
only in a different order:

    while (($key,$value) = each %ENV) {
        print "$key=$value\n";
    }

See also C<keys>, C<values> and C<sort>.

=item eof FILEHANDLE

=item eof ()

=item eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle.  (Note that this function actually
reads a character and then C<ungetc>s it, so isn't very useful in an
interactive context.)  Do not read from a terminal file (or call
C<eof(FILEHANDLE)> on it) after end-of-file is reached.  File types such
as terminals may lose the end-of-file condition if you do.

An C<eof> without an argument uses the last file read as argument.
Using C<eof()> with empty parentheses is very different.  It indicates
the pseudo file formed of the files listed on the command line,
i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
loop to detect the end of only the last file.  Use C<eof(ARGV)> or
C<eof> without the parentheses to test I<each> file in a while
(E<lt>E<gt>) loop.  Examples:

    # reset line numbering on each input file
    while (<>) {
        next if /^\s*#/;        # skip comments 
        print "$.\t$_";
    } continue {
        close ARGV  if eof;     # Not eof()!
    }

    # insert dashes just before last line of last file
    while (<>) {
        if (eof()) {            # check for end of current file
            print "--------------\n";
            close(ARGV);        # close or last; is needed if we
                                # are reading from the terminal
        }
        print;
    }

Practical hint: you almost never need to use C<eof> in Perl, because the
input operators typically return C<undef> when they run out of data, or if
there was an error.

=item eval EXPR

=item eval BLOCK

In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program.  The value of the expression (which is itself
determined within scalar context) is first parsed, and if there weren't any
errors, executed in the context of the current Perl program, so that any
variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the eval executes.  If EXPR is
omitted, evaluates C<$_>.  This form is typically used to delay parsing
and subsequent execution of the text of EXPR until run time.

In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the eval itself was parsed--and executed
within the context of the current Perl program.  This form is typically
used to trap exceptions more efficiently than the first (see below), while
also providing the benefit of checking the code within BLOCK at compile
time.

The final semicolon, if any, may be omitted from the value of EXPR or within
the BLOCK.

In both forms, the value returned is the value of the last expression
evaluated inside the mini-program; a return statement may be also used, just
as with subroutines.  The expression providing the return value is evaluated
in void, scalar, or list context, depending on the context of the eval itself.
See L</wantarray> for more on how the evaluation context can be determined.

If there is a syntax error or runtime error, or a C<die> statement is
executed, an undefined value is returned by C<eval>, and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be a null
string.  Beware that using C<eval> neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility.  See
L</warn> and L<perlvar>.

Note that, because C<eval> traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as C<socket> or C<symlink>)
is implemented.  It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.

If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in C<$@>.
Examples:

    # make divide-by-zero nonfatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = };                 # WRONG

    # a run-time error
    eval '$answer =';   # sets $@

Due to the current arguably broken state of C<__DIE__> hooks, when using
the C<eval{}> form as an exception trap in libraries, you may wish not
to trigger any C<__DIE__> hooks that user code may have installed.
You can use the C<local $SIG{__DIE__}> construct for this purpose,
as shown in this example:

    # a very private exception trap for divide-by-zero
    eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
    warn $@ if $@;

This is especially significant, given that C<__DIE__> hooks can call
C<die> again, which has the effect of changing their error messages:

    # __DIE__ hooks may modify error messages
    {
       local $SIG{'__DIE__'} =
              sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
       eval { die "foo lives here" };
       print $@ if $@;                # prints "bar lives here"
    }

Because this promotes action at a distance, this counterintuitive behavior
may be fixed in a future release.

With an C<eval>, you should be especially careful to remember what's
being looked at when:

    eval $x;            # CASE 1
    eval "$x";          # CASE 2

    eval '$x';          # CASE 3
    eval { $x };        # CASE 4

    eval "\$$x++";      # CASE 5
    $$x++;              # CASE 6

Cases 1 and 2 above behave identically: they run the code contained in
the variable $x.  (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).)  Cases 3
and 4 likewise behave in the same way: they run the code C<'$x'>, which
does nothing but return the value of $x.  (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.)  Case 5 is a place where
normally you I<would> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.

C<eval BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.

=item exec LIST

=item exec PROGRAM LIST

The C<exec> function executes a system command I<and never returns>--
use C<system> instead of C<exec> if you want it to return.  It fails and
returns false only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).

Since it's a common mistake to use C<exec> instead of C<system>, Perl
warns you if there is a following statement which isn't C<die>, C<warn>,
or C<exit> (if C<-w> is set  -  but you always do that).   If you
I<really> want to follow an C<exec> with some other statement, you
can use one of these styles to avoid the warning:

    exec ('foo')   or print STDERR "couldn't exec foo: $!";
    { exec ('foo') }; print STDERR "couldn't exec foo: $!";

If there is more than one argument in LIST, or if LIST is an array
with more than one value, calls execvp(3) with the arguments in LIST.
If there is only one scalar argument or an array with one element in it,
the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing
(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
words and passed directly to C<execvp>, which is more efficient.  
Examples:

    exec '/bin/echo', 'Your arguments are: ', @ARGV;
    exec "sort $outfile | uniq";

If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST.  (This always forces interpretation of the
LIST as a multivalued list, even if there is only a single scalar in
the list.)  Example:

    $shell = '/bin/csh';
    exec $shell '-sh';          # pretend it's a login shell

or, more directly,

    exec {'/bin/csh'} '-sh';    # pretend it's a login shell

When the arguments get executed via the system shell, results will
be subject to its quirks and capabilities.  See L<perlop/"`STRING`">
for details.

Using an indirect object with C<exec> or C<system> is also more
secure.  This usage (which also works fine with system()) forces
interpretation of the arguments as a multivalued list, even if the
list had just one argument.  That way you're safe from the shell
expanding wildcards or splitting up words with whitespace in them.

    @args = ( "echo surprise" );

    exec @args;               # subject to shell escapes
                                # if @args == 1
    exec { $args[0] } @args;  # safe even with one-arg list

The first version, the one without the indirect object, ran the I<echo>
program, passing it C<"surprise"> an argument.  The second version
didn't--it tried to run a program literally called I<"echo surprise">,
didn't find it, and set C<$?> to a non-zero value indicating failure.

Note that C<exec> will not call your C<END> blocks, nor will it call
any C<DESTROY> methods in your objects.

=item exists EXPR

Returns true if the specified hash key exists in its hash, even
if the corresponding value is undefined.

    print "Exists\n"    if exists $array{$key};
    print "Defined\n"   if defined $array{$key};
    print "True\n"      if $array{$key};

A hash element can be true only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.

Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:

    if (exists $ref->{A}->{B}->{$key})  { }
    if (exists $hash{A}{B}{$key})       { }

Although the last element will not spring into existence just because
its existence was tested, intervening ones will.  Thus C<$ref-E<gt>{"A"}>
and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
existence test for a $key element.  This happens anywhere the arrow
operator is used, including even 

    undef $ref;
    if (exists $ref->{"Some key"})      { }
    print $ref;             # prints HASH(0x80d3d5c)

This surprising autovivification in what does not at first--or even
second--glance appear to be an lvalue context may be fixed in a future
release.

See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
used on a pseudo-hash.

=item exit EXPR

Evaluates EXPR and exits immediately with that value.    Example:

    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

See also C<die>.  If EXPR is omitted, exits with C<0> status.  The only
universally recognized values for EXPR are C<0> for success and C<1>
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running.  For example, exiting
69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
the mailer to return the item undelivered, but that's not true everywhere.

Don't use C<exit> to abort a subroutine if there's any chance that
someone might want to trap whatever error happened.  Use C<die> instead,
which can be trapped by an C<eval>.

The exit() function does not always exit immediately.  It calls any
defined C<END> routines first, but these C<END> routines may not
themselves abort the exit.  Likewise any object destructors that need to
be called are called before the real exit.  If this is a problem, you
can call C<POSIX:_exit($status)> to avoid END and destructor processing.
See L<perlmod> for details.

=item exp EXPR

=item exp

Returns I<e> (the natural logarithm base) to the power of EXPR.  
If EXPR is omitted, gives C<exp($_)>.

=item fcntl FILEHANDLE,FUNCTION,SCALAR

Implements the fcntl(2) function.  You'll probably have to say

    use Fcntl;

first to get the correct constant definitions.  Argument processing and
value return works just like C<ioctl> below.  
For example:

    use Fcntl;
    fcntl($filehandle, F_GETFL, $packed_return_buffer)
        or die "can't fcntl F_GETFL: $!";

You don't have to check for C<defined> on the return from C<fnctl>.
Like C<ioctl>, it maps a C<0> return from the system call into
C<"0 but true"> in Perl.  This string is true in boolean context and C<0>
in numeric context.  It is also exempt from the normal B<-w> warnings
on improper numeric conversions.

Note that C<fcntl> will produce a fatal error if used on a machine that
doesn't implement fcntl(2).  See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.

=item fileno FILEHANDLE

Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open.  This is mainly useful for constructing
bitmaps for C<select> and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.

You can use this to find out whether two handles refer to the 
same underlying descriptor:

    if (fileno(THIS) == fileno(THAT)) {
        print "THIS and THAT are dups\n";
    } 

=item flock FILEHANDLE,OPERATION

Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns true
for success, false on failure.  Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
C<flock> is Perl's portable file locking interface, although it locks
only entire files, not records.

Two potentially non-obvious but traditional C<flock> semantics are
that it waits indefinitely until the lock is granted, and that its locks
B<merely advisory>.  Such discretionary locks are more flexible, but offer
fewer guarantees.  This means that files locked with C<flock> may be
modified by programs that do not also use C<flock>.  See L<perlport>,
your port's specific documentation, or your system-specific local manpages
for details.  It's best to assume traditional behavior if you're writing
portable programs.  (But if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies (sometimes called
"features").  Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB.  These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if import them from the Fcntl module,
either individually, or as a group using the ':flock' tag.  LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock.  If LOCK_NB is added to LOCK_SH or
LOCK_EX then C<flock> will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).

To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
before locking or unlocking it.

Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent.  These
are the semantics that lockf(3) implements.  Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.

Note also that some versions of C<flock> cannot lock things over the
network; you would need to use the more system-specific C<fcntl> for
that.  If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch C<-Ud_flock> to the F<Configure> program when you configure
perl.

Here's a mailbox appender for BSD systems.

    use Fcntl ':flock'; # import LOCK_* constants

    sub lock {
        flock(MBOX,LOCK_EX);
        # and, in case someone appended
        # while we were waiting...
        seek(MBOX, 0, 2);
    }

    sub unlock {
        flock(MBOX,LOCK_UN);
    }

    open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
            or die "Can't open mailbox: $!";

    lock();
    print MBOX $msg,"\n\n";
    unlock();

On systems that support a real flock(), locks are inherited across fork()
calls, whereas those that must resort to the more capricious fcntl()
function lose the locks, making it harder to write servers.

See also L<DB_File> for other flock() examples.

=item fork

Does a fork(2) system call to create a new process running the
same program at the same point.  It returns the child pid to the
parent process, C<0> to the child process, or C<undef> if the fork is
unsuccessful.  File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied.  On most systems supporting
fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.

All files opened for output are flushed before forking the child process.

If you C<fork> without ever waiting on your children, you will
accumulate zombies.  On some systems, you can avoid this by setting
C<$SIG{CHLD}> to C<"IGNORE">.  See also L<perlipc> for more examples of
forking and reaping moribund children.

Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, then the remote server (such as, say, a CGI script or a
backgrounded job launched from a remote shell) won't think you're done.
You should reopen those to F</dev/null> if it's any issue.

=item format

Declare a picture format for use by the C<write> function.  For
example:

    format Something =
        Test: @<<<<<<<< @||||| @>>>>>
              $str,     $%,    '$' . int($num)
    .

    $str = "widget";
    $num = $cost/$quantity;
    $~ = 'Something';
    write;

See L<perlform> for many details and examples.

=item formline PICTURE,LIST

This is an internal function used by C<format>s, though you may call it,
too.  It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
Eventually, when a C<write> is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
yourself and then set C<$^A> back to C<"">.  Note that a format typically
does one C<formline> per line of form, but the C<formline> function itself
doesn't care how many newlines are embedded in the PICTURE.  This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.

Be careful if you put double quotes around the picture, because an C<@>
character may be taken to mean the beginning of an array name.
C<formline> always returns true.  See L<perlform> for other examples.

=item getc FILEHANDLE

=item getc

Returns the next character from the input file attached to FILEHANDLE,
or the undefined value at end of file, or if there was an error.
If FILEHANDLE is omitted, reads from STDIN.  This is not particularly
efficient.  However, it cannot be used by itself to fetch single
characters without waiting for the user to hit enter.  For that, try
something more like:

    if ($BSD_STYLE) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", '-icanon', 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD_STYLE) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", 'icanon', 'eol', '^@'; # ASCII null
    }
    print "\n";

Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.

The C<POSIX::getattr> function can do this more portably on
systems purporting POSIX compliance.  See also the C<Term::ReadKey>
module from your nearest CPAN site; details on CPAN can be found on
L<perlmodlib/CPAN>.

=item getlogin

Implements the C library function of the same name, which on most
systems returns the current login from F</etc/utmp>, if any.  If null,
use C<getpwuid>.

    $login = getlogin || getpwuid($<) || "Kilroy";

Do not consider C<getlogin> for authentication: it is not as
secure as C<getpwuid>.

=item getpeername SOCKET

Returns the packed sockaddr address of other end of the SOCKET connection.

    use Socket;
    $hersockaddr    = getpeername(SOCK);
    ($port, $iaddr) = sockaddr_in($hersockaddr);
    $herhostname    = gethostbyaddr($iaddr, AF_INET);
    $herstraddr     = inet_ntoa($iaddr);

=item getpgrp PID

Returns the current process group for the specified PID.  Use
a PID of C<0> to get the current process group for the
current process.  Will raise an exception if used on a machine that
doesn't implement getpgrp(2).  If PID is omitted, returns process
group of current process.  Note that the POSIX version of C<getpgrp>
does not accept a PID argument, so only C<PID==0> is truly portable.

=item getppid

Returns the process id of the parent process.

=item getpriority WHICH,WHO

Returns the current priority for a process, a process group, or a user.
(See L<getpriority(2)>.)  Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).

=item getpwnam NAME

=item getgrnam NAME

=item gethostbyname NAME

=item getnetbyname NAME

=item getprotobyname NAME

=item getpwuid UID

=item getgrgid GID

=item getservbyname NAME,PROTO

=item gethostbyaddr ADDR,ADDRTYPE

=item getnetbyaddr ADDR,ADDRTYPE

=item getprotobynumber NUMBER

=item getservbyport PORT,PROTO

=item getpwent

=item getgrent

=item gethostent

=item getnetent

=item getprotoent

=item getservent

=item setpwent

=item setgrent

=item sethostent STAYOPEN

=item setnetent STAYOPEN

=item setprotoent STAYOPEN

=item setservent STAYOPEN

=item endpwent

=item endgrent

=item endhostent

=item endnetent

=item endprotoent

=item endservent

These routines perform the same functions as their counterparts in the
system library.  In list context, the return values from the
various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
    ($name,$passwd,$gid,$members) = getgr*
    ($name,$aliases,$addrtype,$length,@addrs) = gethost*
    ($name,$aliases,$addrtype,$net) = getnet*
    ($name,$aliases,$proto) = getproto*
    ($name,$aliases,$port,$proto) = getserv*

(If the entry doesn't exist you get a null list.)

In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.)  For example:

    $uid   = getpwnam($name);
    $name  = getpwuid($num);
    $name  = getpwent();
    $gid   = getgrnam($name);
    $name  = getgrgid($num;
    $name  = getgrent();
    #etc.

In I<getpw*()> the fields $quota, $comment, and $expire are
special cases in the sense that in many systems they are unsupported.
If the $quota is unsupported, it is an empty scalar.  If it is
supported, it usually encodes the disk quota.  If the $comment
field is unsupported, it is an empty scalar.  If it is supported it
usually encodes some administrative comment about the user.  In some
systems the $quota field may be $change or $age, fields that have
to do with password aging.  In some systems the $comment field may
be $class.  The $expire field, if present, encodes the expiration
period of the account or the password.  For the availability and the
exact meaning of these fields in your system, please consult your
getpwnam(3) documentation and your F<pwd.h> file.  You can also find
out from within Perl what your $quota and $comment fields mean
and whether you have the $expire field by using the C<Config> module
and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
and C<d_pwexpire>.  Shadow password files are only supported if your
vendor has implemented them in the intuitive fashion that calling the
regular C library routines gets the shadow versions if you're running
under privilege.  Those that incorrectly implement a separate library
call are not supported.

The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.

For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails.  The
C<@addrs> value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call.  In the
Internet domain, each address is four bytes long and you can unpack it
by saying something like:

    ($a,$b,$c,$d) = unpack('C4',$addr[0]);

The Socket library makes this slightly easier:

    use Socket;
    $iaddr = inet_aton("127.1"); # or whatever address
    $name  = gethostbyaddr($iaddr, AF_INET);

    # or going the other way
    $straddr = inet_ntoa($iaddr);

If you get tired of remembering which element of the return list
contains which return value, by-name interfaces are provided
in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
and C<User::grent>.  These override the normal built-ins, supplying
versions that return objects with the appropriate names
for each field.  For example:

   use File::stat;
   use User::pwent;
   $is_his = (stat($filename)->uid == pwent($whoever)->uid);

Even though it looks like they're the same method calls (uid), 
they aren't, because a C<File::stat> object is different from 
a C<User::pwent> object.

=item getsockname SOCKET

Returns the packed sockaddr address of this end of the SOCKET connection,
in case you don't know the address because you have several different
IPs that the connection might have come in on.

    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = sockaddr_in($mysockaddr);
    printf "Connect to %s [%s]\n", 
       scalar gethostbyaddr($myaddr, AF_INET),
       inet_ntoa($myaddr);

=item getsockopt SOCKET,LEVEL,OPTNAME

Returns the socket option requested, or undef if there is an error.

=item glob EXPR

=item glob

Returns the value of EXPR with filename expansions such as the
standard Unix shell F</bin/csh> would do.  This is the internal function
implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
If EXPR is omitted, C<$_> is used.  The C<E<lt>*.cE<gt>> operator is
discussed in more detail in L<perlop/"I/O Operators">.

=item gmtime EXPR

Converts a time as returned by the time function to a 9-element list
with the time localized for the standard Greenwich time zone.
Typically used as follows:

    #  0    1    2     3     4    5     6     7     8
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                            gmtime(time);

All list elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range C<0..11> and $wday
has the range C<0..6> with sunday as day C<0>.  Also, $year is the
number of years since 1900, that is, $year is C<123> in year 2023,
I<not> simply the last two digits of the year.  If you assume it is,
then you create non-Y2K-compliant programs--and you wouldn't want to do
that, would you?

If EXPR is omitted, does C<gmtime(time())>.

In scalar context, returns the ctime(3) value:

    $now_string = gmtime;  # e.g., "Thu Oct 13 04:54:34 1994"

Also see the C<timegm> function provided by the C<Time::Local> module,
and the strftime(3) function available via the POSIX module.

This scalar value is B<not> locale dependent (see L<perllocale>), but
is instead a Perl builtin.  Also see the C<Time::Local> module, and the
strftime(3) and mktime(3) functions available via the POSIX module.  To
get somewhat similar but locale dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>)
and try for example:

    use POSIX qw(strftime);
    $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

Note that the C<%a> and C<%b> escapes, which represent the short forms
of the day of the week and the month of the year, may not necessarily
be three characters wide in all locales.

=item goto LABEL

=item goto EXPR

=item goto &NAME

The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
execution there.  It may not be used to go into any construct that
requires initialization, such as a subroutine or a C<foreach> loop.  It
also can't be used to go into a construct that is optimized away,
or to get out of a block or subroutine given to C<sort>.
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as C<last> or C<die>.  The author of Perl has never felt the
need to use this form of C<goto> (in Perl, that is--C is another matter).

The C<goto-EXPR> form expects a label name, whose scope will be resolved
dynamically.  This allows for computed C<goto>s per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:

    goto ("FOO", "BAR", "GLARCH")[$i];

The C<goto-&NAME> form is quite different from the other forms of C<goto>.
In fact, it isn't a goto in the normal sense at all, and doesn't have
the stigma associated with other gotos.  Instead, it
substitutes a call to the named subroutine for the currently running
subroutine.  This is used by C<AUTOLOAD> subroutines that wish to load
another subroutine and then pretend that the other subroutine had been
called in the first place (except that any modifications to C<@_>
in the current subroutine are propagated to the other subroutine.)
After the C<goto>, not even C<caller> will be able to tell that this
routine was called first.

NAME needn't be the name of a subroutine; it can be a scalar variable
containing a code reference, or a block which evaluates to a code
reference.

=item grep BLOCK LIST

=item grep EXPR,LIST

This is similar in spirit to, but not the same as, grep(1) and its
relatives.  In particular, it is not limited to using regular expressions.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value consisting of those
elements for which the expression evaluated to true.  In scalar
context, returns the number of times the expression was true.

    @foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

    @foo = grep {!/^#/} @bar;    # weed out comments

Note that, because C<$_> is a reference into the list value, it can
be used to modify the elements of the array.  While this is useful and
supported, it can cause bizarre results if the LIST is not a named array.
Similarly, grep returns aliases into the original list, much as a for
loop's index variable aliases the list elements.  That is, modifying an
element of a list returned by grep (for example, in a C<foreach>, C<map>
or another C<grep>) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.

See also L</map> for a list composed of the results of the BLOCK or EXPR.

=item hex EXPR

=item hex

Interprets EXPR as a hex string and returns the corresponding value.
(To convert strings that might start with either 0, 0x, or 0b, see
L</oct>.)  If EXPR is omitted, uses C<$_>.

    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same

Hex strings may only represent integers.  Strings that would cause
integer overflow trigger a warning.

=item import

There is no builtin C<import> function.  It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module.  The C<use> function calls the C<import> method
for the package used.  See also L</use()>, L<perlmod>, and L<Exporter>.

=item index STR,SUBSTR,POSITION

=item index STR,SUBSTR

The index function searches for one string within another, but without
the wildcard-like behavior of a full regular-expression pattern match.
It returns the position of the first occurrence of SUBSTR in STR at
or after POSITION.  If POSITION is omitted, starts searching from the
beginning of the string.  The return value is based at C<0> (or whatever
you've set the C<$[> variable to--but don't do that).  If the substring
is not found, returns one less than the base, ordinarily C<-1>.

=item int EXPR

=item int

Returns the integer portion of EXPR.  If EXPR is omitted, uses C<$_>.
You should not use this function for rounding: one because it truncates
towards C<0>, and two because machine representations of floating point
numbers can sometimes produce counterintuitive results.  For example,
C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
because it's really more like -268.99999999999994315658 instead.  Usually,
the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
functions will serve you better than will int().

=item ioctl FILEHANDLE,FUNCTION,SCALAR

Implements the ioctl(2) function.  You'll probably first have to say

    require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph

to get the correct function definitions.  If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
(There is a Perl script called B<h2ph> that comes with the Perl kit that
may help you in this, but it's nontrivial.)  SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
will be passed as the third argument of the actual C<ioctl> call.  (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value.  To guarantee this to be
true, add a C<0> to the scalar before using it.)  The C<pack> and C<unpack>
functions may be needed to manipulate the values of structures used by
C<ioctl>.  

The return value of C<ioctl> (and C<fcntl>) is as follows:

        if OS returns:          then Perl returns:
            -1                    undefined value
             0                  string "0 but true"
        anything else               that number

Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:

    $retval = ioctl(...) || -1;
    printf "System returned %d\n", $retval;

The special string "C<0> but true" is exempt from B<-w> complaints
about improper numeric conversions.

Here's an example of setting a filehandle named C<REMOTE> to be
non-blocking at the system level.  You'll have to negotiate C<$|>
on your own, though.

    use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

    $flags = fcntl(REMOTE, F_GETFL, 0)
                or die "Can't get flags for the socket: $!\n";

    $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
                or die "Can't set flags for the socket: $!\n";

=item join EXPR,LIST

Joins the separate strings of LIST into a single string with fields
separated by the value of EXPR, and returns that new string.  Example:

    $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

Beware that unlike C<split>, C<join> doesn't take a pattern as its
first argument.  Compare L</split>.

=item keys HASH

Returns a list consisting of all the keys of the named hash.  (In
scalar context, returns the number of keys.)  The keys are returned in
an apparently random order.  The actual random order is subject to
change in future versions of perl, but it is guaranteed to be the same
order as either the C<values> or C<each> function produces (given
that the hash has not been modified).  As a side effect, it resets
HASH's iterator.

Here is yet another way to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while (@keys) { 
        print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

    foreach $key (sort(keys %ENV)) {
        print $key, '=', $ENV{$key}, "\n";
    }

To sort a hash by value, you'll need to use a C<sort> function.
Here's a descending numeric sort of a hash by its values:

    foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
        printf "%4d %s\n", $hash{$key}, $key;
    }

As an lvalue C<keys> allows you to increase the number of hash buckets
allocated for the given hash.  This can gain you a measure of efficiency if
you know the hash is going to get big.  (This is similar to pre-extending
an array by assigning a larger number to $#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it--256 of them,
in fact, since it rounds up to the next power of two.  These
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

See also C<each>, C<values> and C<sort>.

=item kill SIGNAL, LIST

Sends a signal to a list of processes.  Returns the number of
processes successfully signaled (which is not necessarily the
same as the number actually killed).

    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;

If SIGNAL is zero, no signal is sent to the process.  This is a
useful way to check that the process is alive and hasn't changed
its UID.  See L<perlport> for notes on the portability of this
construct.

Unlike in the shell, if SIGNAL is negative, it kills
process groups instead of processes.  (On System V, a negative I<PROCESS>
number will also kill process groups, but that's not portable.)  That
means you usually want to use positive not negative signals.  You may also
use a signal name in quotes.  See L<perlipc/"Signals"> for details.

=item last LABEL

=item last

The C<last> command is like the C<break> statement in C (as used in
loops); it immediately exits the loop in question.  If the LABEL is
omitted, the command refers to the innermost enclosing loop.  The
C<continue> block, if any, is not executed:

    LINE: while (<STDIN>) {
        last LINE if /^$/;      # exit when done with header
        #...
    }

C<last> cannot be used to exit a block which returns a value such as
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.

Note that a block by itself is semantically identical to a loop
that executes once.  Thus C<last> can be used to effect an early
exit out of such a block.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item lc EXPR

=item lc

Returns an lowercased version of EXPR.  This is the internal function
implementing the C<\L> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>
and L<utf8>.

If EXPR is omitted, uses C<$_>.

=item lcfirst EXPR

=item lcfirst

Returns the value of EXPR with the first character lowercased.  This is
the internal function implementing the C<\l> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses C<$_>.

=item length EXPR

=item length

Returns the length in characters of the value of EXPR.  If EXPR is
omitted, returns length of C<$_>.  Note that this cannot be used on 
an entire array or hash to find out how many elements these have.
For that, use C<scalar @array> and C<scalar keys %hash> respectively.

=item link OLDFILE,NEWFILE

Creates a new filename linked to the old filename.  Returns true for
success, false otherwise. 

=item listen SOCKET,QUEUESIZE

Does the same thing that the listen system call does.  Returns true if
it succeeded, false otherwise.  See the example in L<perlipc/"Sockets: Client/Server Communication">.

=item local EXPR

You really probably want to be using C<my> instead, because C<local> isn't
what most people think of as "local".  See L<perlsub/"Private Variables
via my()"> for details.

A local modifies the listed variables to be local to the enclosing
block, file, or eval.  If more than one value is listed, the list must
be placed in parentheses.  See L<perlsub/"Temporary Values via local()">
for details, including issues with tied arrays and hashes.

=item localtime EXPR

Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone.  Typically used as
follows:

    #  0    1    2     3     4    5     6     7     8
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                localtime(time);

All list elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range C<0..11> and $wday
has the range C<0..6> with sunday as day C<0>.  Also, $year is the
number of years since 1900, that is, $year is C<123> in year 2023,
and I<not> simply the last two digits of the year.  If you assume it is,
then you create non-Y2K-compliant programs--and you wouldn't want to do
that, would you?

If EXPR is omitted, uses the current time (C<localtime(time)>).

In scalar context, returns the ctime(3) value:

    $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

This scalar value is B<not> locale dependent, see L<perllocale>, but
instead a Perl builtin.  Also see the C<Time::Local> module
(to convert the second, minutes, hours, ... back to seconds since the
stroke of midnight the 1st of January 1970, the value returned by
time()), and the strftime(3) and mktime(3) function available via the
POSIX module.  To get somewhat similar but locale dependent date
strings, set up your locale environment variables appropriately
(please see L<perllocale>) and try for example:

    use POSIX qw(strftime);
    $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;

Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.

=item lock

    lock I<THING>

This function places an advisory lock on a variable, subroutine,
or referenced object contained in I<THING> until the lock goes out
of scope.  This is a built-in function only if your version of Perl
was built with threading enabled, and if you've said C<use Threads>.
Otherwise a user-defined function by this name will be called.  See
L<Thread>.

=item log EXPR

=item log

Returns the natural logarithm (base I<e>) of EXPR.  If EXPR is omitted,
returns log of C<$_>.  To get the log of another base, use basic algebra:
The base-N log of a number is equal to the natural log of that number
divided by the natural log of N.  For example:

    sub log10 {
        my $n = shift;
        return log($n)/log(10);
    } 

See also L</exp> for the inverse operation.

=item lstat FILEHANDLE

=item lstat EXPR

=item lstat

Does the same thing as the C<stat> function (including setting the
special C<_> filehandle) but stats a symbolic link instead of the file
the symbolic link points to.  If symbolic links are unimplemented on
your system, a normal C<stat> is done.

If EXPR is omitted, stats C<$_>.

=item m//

The match operator.  See L<perlop>.

=item map BLOCK LIST

=item map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value composed of the
results of each such evaluation.  In scalar context, returns the
total number of elements so generated.  Evaluates BLOCK or EXPR in
list context, so each element of LIST may produce zero, one, or
more elements in the returned value.

    @chars = map(chr, @nums);

translates a list of numbers to the corresponding characters.  And

    %hash = map { getkey($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach $_ (@array) {
        $hash{getkey($_)} = $_;
    }

Note that, because C<$_> is a reference into the list value, it can
be used to modify the elements of the array.  While this is useful and
supported, it can cause bizarre results if the LIST is not a named array.
Using a regular C<foreach> loop for this purpose would be clearer in
most cases.  See also L</grep> for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.

=item mkdir FILENAME,MASK

Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by C<umask>).  If it succeeds it
returns true, otherwise it returns false and sets C<$!> (errno).

In general, it is better to create directories with permissive MASK,
and let the user modify that with their C<umask>, than it is to supply
a restrictive MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance).  The perlfunc(1) entry on
C<umask> discusses the choice of MASK in more detail.

=item msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2).  You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions.  If CMD is C<IPC_STAT>,
then ARG must be a variable which will hold the returned C<msqid_ds>
structure.  Returns like C<ioctl>: the undefined value for error,
C<"0 but true"> for zero, or the actual return value otherwise.  See also
C<IPC::SysV> and C<IPC::Semaphore> documentation.

=item msgget KEY,FLAGS

Calls the System V IPC function msgget(2).  Returns the message queue
id, or the undefined value if there is an error.  See also C<IPC::SysV>
and C<IPC::Msg> documentation.

=item msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID.  MSG must begin with the long integer message type,
which may be created with C<pack("l", $type)>.  Returns true if
successful, or false if there is an error.  See also C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.

=item msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE.  Note that if a message is received, the message type will be
the first thing in VAR, and the maximum length of VAR is SIZE plus the
size of the message type.  Returns true if successful, or false if
there is an error.  See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.

=item my EXPR

=item my EXPR : ATTRIBUTES

A C<my> declares the listed variables to be local (lexically) to the
enclosing block, file, or C<eval>.  If
more than one value is listed, the list must be placed in parentheses.  See
L<perlsub/"Private Variables via my()"> for details.

=item next LABEL

=item next

The C<next> command is like the C<continue> statement in C; it starts
the next iteration of the loop:

    LINE: while (<STDIN>) {
        next LINE if /^#/;      # discard comments
        #...
    }

Note that if there were a C<continue> block on the above, it would get
executed even on discarded lines.  If the LABEL is omitted, the command
refers to the innermost enclosing loop.

C<next> cannot be used to exit a block which returns a value such as
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.

Note that a block by itself is semantically identical to a loop
that executes once.  Thus C<next> will exit such a block early.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item no Module LIST

See the L</use> function, which C<no> is the opposite of.

=item oct EXPR

=item oct

Interprets EXPR as an octal string and returns the corresponding
value.  (If EXPR happens to start off with C<0x>, interprets it as a
hex string.  If EXPR starts off with C<0b>, it is interpreted as a
binary string.)  The following will handle decimal, binary, octal, and
hex in the standard Perl or C notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses C<$_>.   To go the other way (produce a number
in octal), use sprintf() or printf():

    $perms = (stat("filename"))[2] & 07777;
    $oct_perms = sprintf "%lo", $perms;

The oct() function is commonly used when a string such as C<644> needs
to be converted into a file mode, for example. (Although perl will
automatically convert strings into numbers as needed, this automatic
conversion assumes base 10.)

=item open FILEHANDLE,MODE,EXPR

=item open FILEHANDLE,EXPR

=item open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.  If FILEHANDLE is an expression, its value is used as the
name of the real filehandle wanted.  If EXPR is omitted, the scalar
variable of the same name as the FILEHANDLE contains the filename.
(Note that lexical variables--those declared with C<my>--will not work
for this purpose; so if you're using C<my>, specify EXPR in your call
to open.)  See L<perlopentut> for a kinder, gentler explanation of opening
files.

If MODE is C<'E<lt>'> or nothing, the file is opened for input.
If MODE is C<'E<gt>'>, the file is truncated and opened for
output, being created if necessary.  If MODE is C<'E<gt>E<gt>'>,
the file is opened for appending, again being created if necessary. 
You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
you want both read and write access to the file; thus C<'+E<lt>'> is almost
always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
file first.  You can't usually use either read-write mode for updating
textfiles, since they have variable length records.  See the B<-i>
switch in L<perlrun> for a better approach.  The file is created with
permissions of C<0666> modified by the process' C<umask> value.

These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
C<'w+'>, C<'a'>, and C<'a+'>.

In the 2-arguments (and 1-argument) form of the call the mode and
filename should be concatenated (in this order), possibly separated by
spaces.  It is possible to omit the mode if the mode is C<'E<lt>'>.

If the filename begins with C<'|'>, the filename is interpreted as a
command to which output is to be piped, and if the filename ends with a
C<'|'>, the filename is interpreted as a command which pipes output to
us.  See L<perlipc/"Using open() for IPC">
for more examples of this.  (You are not allowed to C<open> to a command
that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
and L<perlipc/"Bidirectional Communication"> for alternatives.)

If MODE is C<'|-'>, the filename is interpreted as a
command to which output is to be piped, and if MODE is
C<'-|'>, the filename is interpreted as a command which pipes output to
us.  In the 2-arguments (and 1-argument) form one should replace dash
(C<'-'>) with the command.  See L<perlipc/"Using open() for IPC">
for more examples of this.  (You are not allowed to C<open> to a command
that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
and L<perlipc/"Bidirectional Communication"> for alternatives.)

In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
and opening C<'E<gt>-'> opens STDOUT.  

Open returns
nonzero upon success, the undefined value otherwise.  If the C<open>
involved a pipe, the return value happens to be the pid of the
subprocess.

If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</binmode> for tips for
dealing with this.  The key distinction between systems that need C<binmode>
and those that don't is their text file formats.  Systems like Unix, MacOS, and
Plan9, which delimit lines with a single character, and which encode that
character in C as C<"\n">, do not need C<binmode>.  The rest need it.

When opening a file, it's usually a bad idea to continue normal execution
if the request failed, so C<open> is frequently used in connection with
C<die>.  Even if C<die> won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
modules that can help with that problem)) you should always check
the return value from opening a file.  The infrequent exception is when
working with an unopened filehandle is actually what you want to do.

Examples:

    $ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, '>>/usr/spool/news/twitlog');     # (log is reserved)
    # if the open fails, output is discarded

    open(DBASE, '+<', 'dbase.mine')             # open for update
        or die "Can't open 'dbase.mine' for update: $!";

    open(DBASE, '+<dbase.mine')                 # ditto
        or die "Can't open 'dbase.mine' for update: $!";

    open(ARTICLE, '-|', "caesar <$article")     # decrypt article
        or die "Can't start caesar: $!";

    open(ARTICLE, "caesar <$article |")         # ditto
        or die "Can't start caesar: $!";

    open(EXTRACT, "|sort >/tmp/Tmp$$")          # $$ is our process id
        or die "Can't start sort: $!";

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
        process($file, 'fh00');
    }

    sub process {
        my($filename, $input) = @_;
        $input++;               # this is a string increment
        unless (open($input, $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }

        local $_;
        while (<$input>) {              # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            #...                # whatever
        }
    }

You may also, in the Bourne shell tradition, specify an EXPR beginning
with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) to be
duped and opened.  You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>.  The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
stdio buffers.)  Duping file handles is not yet supported for 3-argument
open().

Here is a script that saves, redirects, and restores STDOUT and
STDERR:

    #!/usr/bin/perl
    open(OLDOUT, ">&STDOUT");
    open(OLDERR, ">&STDERR");

    open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
    open(STDERR, ">&STDOUT")     || die "Can't dup stdout";

    select(STDERR); $| = 1;     # make unbuffered
    select(STDOUT); $| = 1;     # make unbuffered

    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too

    close(STDOUT);
    close(STDERR);

    open(STDOUT, ">&OLDOUT");
    open(STDERR, ">&OLDERR");

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";

If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
equivalent of C's C<fdopen> of that file descriptor; this is more
parsimonious of file descriptors.  For example:

    open(FILEHANDLE, "<&=$fd")

Note that this feature depends on the fdopen() C library function.
On many UNIX systems, fdopen() is known to fail when file descriptors
exceed a certain value, typically 255. If you need more file
descriptors than that, consider rebuilding Perl to use the C<sfio>
library.

If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
with 2-arguments (or 1-argument) form of open(), then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and C<0> within the child
process.  (Use C<defined($pid)> to determine whether the open was successful.)
The filehandle behaves normally for the parent, but i/o to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--i/o happens from/to
the new STDOUT or STDIN.  Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.
The following triples are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
    open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';

    open(FOO, "cat -n '$file'|");
    open(FOO, '-|', "cat -n '$file'");
    open(FOO, '-|') || exec 'cat', '-n', $file;

See L<perlipc/"Safe Pipe Opens"> for more examples of this.

NOTE: On any operation that may do a fork, all files opened for output
are flushed before the fork is attempted.  On systems that support a
close-on-exec flag on files, the flag will be set for the newly opened
file descriptor as determined by the value of $^F.  See L<perlvar/$^F>.

Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?>.

The filename passed to 2-argument (or 1-argument) form of open()
will have leading and trailing
whitespace deleted, and the normal redirection characters
honored.  This property, known as "magic open", 
can often be used to good effect.  A user could specify a filename of
F<"rsh cat file |">, or you could change certain filenames as needed:

    $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    open(FH, $filename) or die "Can't open $filename: $!";

Use 3-argument form to open a file with arbitrary weird characters in it,

    open(FOO, '<', $file);

otherwise it's necessary to protect any leading and trailing whitespace:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

(this may not work on some bizzare filesystems).  One should
conscientiously choose between the the I<magic> and 3-arguments form
of open():

    open IN, $ARGV[0];

will allow the user to specify an argument of the form C<"rsh cat file |">,
but will not work on a filename which happens to have a trailing space, while

    open IN, '<', $ARGV[0];

will have exactly the opposite restrictions.

If you want a "real" C C<open> (see L<open(2)> on your system), then you
should use the C<sysopen> function, which involves no such magic (but
may use subtly different filemodes than Perl open(), which is mapped
to C fopen()).  This is
another way to protect your filenames from interpretation.  For example:

    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
        or die "sysopen $path: $!";
    $oldfh = select(HANDLE); $| = 1; select($oldfh);
    print HANDLE "stuff $$\n");
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

Using the constructor from the C<IO::Handle> package (or one of its
subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
filehandles that have the scope of whatever variables hold references to
them, and automatically close whenever and however you leave that scope:

    use IO::File;
    #...
    sub read_myfile_munged {
        my $ALL = shift;
        my $handle = new IO::File;
        open($handle, "myfile") or die "myfile: $!";
        $first = <$handle>
            or return ();     # Automatically closed here.
        mung $first or die "mung failed";       # Or here.
        return $first, <$handle> if $ALL;       # Or here.
        $first;                                 # Or here.
    }

See L</seek> for some details about mixing reading and writing.

=item opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
C<seekdir>, C<rewinddir>, and C<closedir>.  Returns true if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.

=item ord EXPR

=item ord

Returns the numeric (ASCII or Unicode) value of the first character of EXPR.  If
EXPR is omitted, uses C<$_>.  For the reverse, see L</chr>.
See L<utf8> for more about Unicode.

=item our EXPR

An C<our> declares the listed variables to be valid globals within
the enclosing block, file, or C<eval>.  That is, it has the same
scoping rules as a "my" declaration, but does not create a local
variable.  If more than one value is listed, the list must be placed
in parentheses.  The C<our> declaration has no semantic effect unless
"use strict vars" is in effect, in which case it lets you use the
declared global variable without qualifying it with a package name.
(But only within the lexical scope of the C<our> declaration.  In this
it differs from "use vars", which is package scoped.)

=item pack TEMPLATE,LIST

Takes a LIST of values and converts it into a string using the rules
given by the TEMPLATE.  The resulting string is the concatenation of
the converted values.  Typically, each converted value looks
like its machine-level representation.  For example, on 32-bit machines
a converted integer may be represented by a sequence of 4 bytes.

The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:

    a   A string with arbitrary binary data, will be null padded.
    A   An ascii string, will be space padded.
    Z   A null terminated (asciz) string, will be null padded.

    b   A bit string (ascending bit order inside each byte, like vec()).
    B   A bit string (descending bit order inside each byte).
    h   A hex string (low nybble first).
    H   A hex string (high nybble first).

    c   A signed char value.
    C   An unsigned char value.  Only does bytes.  See U for Unicode.

    s   A signed short value.
    S   An unsigned short value.
          (This 'short' is _exactly_ 16 bits, which may differ from
           what a local C compiler calls 'short'.  If you want
           native-length shorts, use the '!' suffix.)

    i   A signed integer value.
    I   An unsigned integer value.
          (This 'integer' is _at_least_ 32 bits wide.  Its exact
           size depends on what a local C compiler calls 'int',
           and may even be larger than the 'long' described in
           the next item.)

    l   A signed long value.
    L   An unsigned long value.
          (This 'long' is _exactly_ 32 bits, which may differ from
           what a local C compiler calls 'long'.  If you want
           native-length longs, use the '!' suffix.)

    n   An unsigned short in "network" (big-endian) order.
    N   An unsigned long in "network" (big-endian) order.
    v   An unsigned short in "VAX" (little-endian) order.
    V   An unsigned long in "VAX" (little-endian) order.
          (These 'shorts' and 'longs' are _exactly_ 16 bits and
           _exactly_ 32 bits, respectively.)

    q   A signed quad (64-bit) value.
    Q   An unsigned quad value.
          (Quads are available only if your system supports 64-bit
           integer values _and_ if Perl has been compiled to support those.
           Causes a fatal error otherwise.)

    f   A single-precision float in the native format.
    d   A double-precision float in the native format.

    p   A pointer to a null-terminated string.
    P   A pointer to a structure (fixed-length string).

    u   A uuencoded string.
    U   A Unicode character number.  Encodes to UTF-8 internally.
        Works even if C<use utf8> is not in effect.

    w   A BER compressed integer.  Its bytes represent an unsigned
        integer in base 128, most significant digit first, with as
        few digits as possible.  Bit eight (the high bit) is set
        on each byte except the last.

    x   A null byte.
    X   Back up a byte.
    @   Null fill to absolute position.

The following rules apply:

=over 8

=item *

Each letter may optionally be followed by a number giving a repeat
count.  With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
C<H>, and C<P> the pack function will gobble up that many values from
the LIST.  A C<*> for the repeat count means to use however many items are
left, except for C<@>, C<x>, C<X>, where it is equivalent
to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
same).

When used with C<Z>, C<*> results in the addition of a trailing null
byte (so the packed result will be one longer than the byte C<length>
of the item).

The repeat count for C<u> is interpreted as the maximal number of bytes
to encode per line of output, with 0 and 1 replaced by 45.

=item *

The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
string of length count, padding with nulls or spaces as necessary.  When
unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
after the first null, and C<a> returns data verbatim.  When packing,
C<a>, and C<Z> are equivalent.

If the value-to-pack is too long, it is truncated.  If too long and an
explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
by a null byte.  Thus C<Z> always packs a trailing null byte under
all circumstances.

=item *

Likewise, the C<b> and C<B> fields pack a string that many bits long.
Each byte of the input field of pack() generates 1 bit of the result.
Each result bit is based on the least-significant bit of the corresponding
input byte, i.e., on C<ord($byte)%2>.  In particular, bytes C<"0"> and
C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.

Starting from the beginning of the input string of pack(), each 8-tuple
of bytes is converted to 1 byte of output.  With format C<b>
the first byte of the 8-tuple determines the least-significant bit of a
byte, and with format C<B> it determines the most-significant bit of
a byte.

If the length of the input string is not exactly divisible by 8, the
remainder is packed as if the input string were padded by null bytes
at the end.  Similarly, during unpack()ing the "extra" bits are ignored.

If the input string of pack() is longer than needed, extra bytes are ignored.
A C<*> for the repeat count of pack() means to use all the bytes of
the input field.  On unpack()ing the bits are converted to a string
of C<"0">s and C<"1">s.

=item *

The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
representable as hexadecimal digits, 0-9a-f) long.

Each byte of the input field of pack() generates 4 bits of the result.
For non-alphabetical bytes the result is based on the 4 least-significant
bits of the input byte, i.e., on C<ord($byte)%16>.  In particular,
bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
C<"\0"> and C<"\1">.  For bytes C<"a".."f"> and C<"A".."F"> the result
is compatible with the usual hexadecimal digits, so that C<"a"> and
C<"A"> both generate the nybble C<0xa==10>.  The result for bytes
C<"g".."z"> and C<"G".."Z"> is not well-defined.

Starting from the beginning of the input string of pack(), each pair
of bytes is converted to 1 byte of output.  With format C<h> the
first byte of the pair determines the least-significant nybble of the
output byte, and with format C<H> it determines the most-significant
nybble.

If the length of the input string is not even, it behaves as if padded
by a null byte at the end.  Similarly, during unpack()ing the "extra"
nybbles are ignored.

If the input string of pack() is longer than needed, extra bytes are ignored.
A C<*> for the repeat count of pack() means to use all the bytes of
the input field.  On unpack()ing the bits are converted to a string
of hexadecimal digits.

=item *

The C<p> type packs a pointer to a null-terminated string.  You are
responsible for ensuring the string is not a temporary value (which can
potentially get deallocated before you get around to using the packed result).
The C<P> type packs a pointer to a structure of the size indicated by the
length.  A NULL pointer is created if the corresponding value for C<p> or
C<P> is C<undef>, similarly for unpack().

=item *

The C</> template character allows packing and unpacking of strings where
the packed structure contains a byte count followed by the string itself.
You write I<length-item>C</>I<string-item>.

The I<length-item> can be any C<pack> template letter,
and describes how the length value is packed.
The ones likely to be of most use are integer-packing ones like
C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
and C<N> (for Sun XDR).

The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
For C<unpack> the length of the string is obtained from the I<length-item>,
but if you put in the '*' it will be ignored.

    unpack 'C/a', "\04Gurusamy";        gives 'Guru'
    unpack 'a3/A* A*', '007 Bond  J ';  gives (' Bond','J')
    pack 'n/a* w/a*','hello,','world';  gives "\000\006hello,\005world"

The I<length-item> is not returned explicitly from C<unpack>.

Adding a count to the I<length-item> letter is unlikely to do anything
useful, unless that letter is C<A>, C<a> or C<Z>.  Packing with a
I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
which Perl does not regard as legal in numeric strings.

=item *

The integer types C<s>, C<S>, C<l>, and C<L> may be
immediately followed by a C<!> suffix to signify native shorts or
longs--as you can see from above for example a bare C<l> does mean
exactly 32 bits, the native C<long> (as seen by the local C compiler)
may be larger.  This is an issue mainly in 64-bit platforms.  You can
see whether using C<!> makes any difference by

        print length(pack("s")), " ", length(pack("s!")), "\n";
        print length(pack("l")), " ", length(pack("l!")), "\n";

C<i!> and C<I!> also work but only because of completeness;
they are identical to C<i> and C<I>.

The actual sizes (in bytes) of native shorts, ints, longs, and long
longs on the platform where Perl was built are also available via
L<Config>:

       use Config;
       print $Config{shortsize},    "\n";
       print $Config{intsize},      "\n";
       print $Config{longsize},     "\n";
       print $Config{longlongsize}, "\n";

(The C<$Config{longlongsize}> will be undefine if your system does
not support long longs.) 

=item *

The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
are inherently non-portable between processors and operating systems
because they obey the native byteorder and endianness.  For example a
4-byte integer 0x12345678 (305419896 decimal) be ordered natively
(arranged in and handled by the CPU registers) into bytes as
 
        0x12 0x34 0x56 0x78     # little-endian
        0x78 0x56 0x34 0x12     # big-endian
 
Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
Power, and Cray are big-endian.  MIPS can be either: Digital used it
in little-endian mode; SGI uses it in big-endian mode.

The names `big-endian' and `little-endian' are comic references to
the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
the egg-eating habits of the Lilliputians.
 
Some systems may have even weirder byte orders such as
 
        0x56 0x78 0x12 0x34
        0x34 0x12 0x78 0x56
 
You can see your system's preference with

        print join(" ", map { sprintf "%#02x", $_ }
                            unpack("C*",pack("L",0x12345678))), "\n";

The byteorder on the platform where Perl was built is also available
via L<Config>:

        use Config;
        print $Config{byteorder}, "\n";

Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
and C<'87654321'> are big-endian.

If you want portable packed integers use the formats C<n>, C<N>,
C<v>, and C<V>, their byte endianness and size is known.
See also L<perlport>.

=item *

Real numbers (floats and doubles) are in the native machine format only;
due to the multiplicity of floating formats around, and the lack of a
standard "network" representation, no facility for interchange has been
made.  This means that packed floating point data written on one machine
may not be readable on another - even if both use IEEE floating point
arithmetic (as the endian-ness of the memory representation is not part
of the IEEE spec).  See also L<perlport>.

Note that Perl uses doubles internally for all numeric calculation, and
converting from double into float and thence back to double again will
lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
equal $foo).

=item *

You must yourself do any alignment or padding by inserting for example
enough C<'x'>es while packing.  There is no way to pack() and unpack()
could know where the bytes are going to or coming from.  Therefore
C<pack> (and C<unpack>) handle their output and input as flat
sequences of bytes.

=item *

A comment in a TEMPLATE starts with C<#> and goes to the end of line.

=item *

If TEMPLATE requires more arguments to pack() than actually given, pack()
assumes additional C<""> arguments.  If TEMPLATE requires less arguments
to pack() than actually given, extra arguments are ignored.

=back

Examples:

    $foo = pack("CCCC",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("C4",65,66,67,68);
    # same thing
    $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    # note: the above examples featuring "C" and "c" are true
    # only on ASCII and ASCII-derived systems such as ISO Latin 1
    # and UTF-8.  In EBCDIC the first example would be
    # $foo = pack("CCCC",193,194,195,196);

    $foo = pack("s2",1,2);
    # "\1\0\2\0" on little-endian
    # "\0\1\0\2" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    $utmp_template = "Z8 Z8 Z16 L";
    $utmp = pack($utmp_template, @utmp1);
    # a struct utmp (BSDish)

    @utmp2 = unpack($utmp_template, $utmp);
    # "@utmp1" eq "@utmp2"

    sub bintodec {
        unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

    $foo = pack('sx2l', 12, 34);
    # short 12, two zero bytes padding, long 34
    $bar = pack('s@4l', 12, 34);
    # short 12, zero fill to position 4, long 34
    # $foo eq $bar

The same template may generally also be used in unpack().

=item package 

=item package NAMESPACE

Declares the compilation unit as being in the given namespace.  The scope
of the package declaration is from the declaration itself through the end
of the enclosing block, file, or eval (the same as the C<my> operator).
All further unqualified dynamic identifiers will be in this namespace.
A package statement affects only dynamic variables--including those
you've used C<local> on--but I<not> lexical variables, which are created
with C<my>.  Typically it would be the first declaration in a file to
be included by the C<require> or C<use> operator.  You can switch into a
package in more than one place; it merely influences which symbol table
is used by the compiler for the rest of that block.  You can refer to
variables and filehandles in other packages by prefixing the identifier
with the package name and a double colon:  C<$Package::Variable>.
If the package name is null, the C<main> package as assumed.  That is,
C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
still seen in older code).

If NAMESPACE is omitted, then there is no current package, and all
identifiers must be fully qualified or lexicals.  This is stricter
than C<use strict>, since it also extends to function names.

See L<perlmod/"Packages"> for more information about packages, modules,
and classes.  See L<perlsub> for other scoping issues.

=item pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful.  In addition, note that Perl's pipes use
stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.

See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.

On systems that support a close-on-exec flag on files, the flag will be set
for the newly opened file descriptors as determined by the value of $^F.
See L<perlvar/$^F>.

=item pop ARRAY

=item pop

Pops and returns the last value of the array, shortening the array by
one element.  Has an effect similar to

    $ARRAY[$#ARRAY--]

If there are no elements in the array, returns the undefined value
(although this may happen at other times as well).  If ARRAY is
omitted, pops the C<@ARGV> array in the main program, and the C<@_>
array in subroutines, just like C<shift>.

=item pos SCALAR

=item pos

Returns the offset of where the last C<m//g> search left off for the variable
is in question (C<$_> is used when the variable is not specified).  May be
modified to change that offset.  Such modification will also influence
the C<\G> zero-width assertion in regular expressions.  See L<perlre> and
L<perlop>.

=item print FILEHANDLE LIST

=item print LIST

=item print

Prints a string or a list of strings.  Returns true if successful.
FILEHANDLE may be a scalar variable name, in which case the variable
contains the name of or a reference to the filehandle, thus introducing
one level of indirection.  (NOTE: If FILEHANDLE is a variable and
the next token is a term, it may be misinterpreted as an operator
unless you interpose a C<+> or put parentheses around the arguments.)
If FILEHANDLE is omitted, prints by default to standard output (or
to the last selected output channel--see L</select>).  If LIST is
also omitted, prints C<$_> to the currently selected output channel.
To set the default output channel to something other than STDOUT
use the select operation.  The current value of C<$,> (if any) is
printed between each LIST item.  The current value of C<$\> (if
any) is printed after the entire LIST has been printed.  Because
print takes a LIST, anything in the LIST is evaluated in list
context, and any subroutine that you call will have one or more of
its expressions evaluated in list context.  Also be careful not to
follow the print keyword with a left parenthesis unless you want
the corresponding right parenthesis to terminate the arguments to
the print--interpose a C<+> or put parentheses around all the
arguments.

Note that if you're storing FILEHANDLES in an array or other expression,
you will have to use a block returning its value instead:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";

=item printf FILEHANDLE FORMAT, LIST

=item printf FORMAT, LIST

Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
(the output record separator) is not appended.  The first argument
of the list will be interpreted as the C<printf> format.  If C<use locale> is
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale.  See L<perllocale>.

Don't fall into the trap of using a C<printf> when a simple
C<print> would do.  The C<print> is more efficient and less
error prone.

=item prototype FUNCTION

Returns the prototype of a function as a string (or C<undef> if the
function has no prototype).  FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve.

If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
name for Perl builtin.  If the builtin is not I<overridable> (such as
C<qw//>) or its arguments cannot be expressed by a prototype (such as
C<system>) returns C<undef> because the builtin does not really behave
like a Perl function.  Otherwise, the string describing the equivalent
prototype is returned.

=item push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST
onto the end of ARRAY.  The length of ARRAY increases by the length of
LIST.  Has the same effect as

    for $value (LIST) {
        $ARRAY[++$#ARRAY] = $value;
    }

but is more efficient.  Returns the new number of elements in the array.

=item q/STRING/

=item qq/STRING/

=item qr/STRING/

=item qx/STRING/

=item qw/STRING/

Generalized quotes.  See L<perlop/"Regexp Quote-Like Operators">.

=item quotemeta EXPR

=item quotemeta

Returns the value of EXPR with all non-alphanumeric
characters backslashed.  (That is, all characters not matching
C</[A-Za-z_0-9]/> will be preceded by a backslash in the
returned string, regardless of any locale settings.)
This is the internal function implementing
the C<\Q> escape in double-quoted strings.

If EXPR is omitted, uses C<$_>.

=item rand EXPR

=item rand

Returns a random fractional number greater than or equal to C<0> and less
than the value of EXPR.  (EXPR should be positive.)  If EXPR is
omitted, the value C<1> is used.  Automatically calls C<srand> unless
C<srand> has already been called.  See also C<srand>.

(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
with the wrong number of RANDBITS.)

=item read FILEHANDLE,SCALAR,LENGTH,OFFSET

=item read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE.  Returns the number of bytes actually read,
C<0> at end of file, or undef if there was an error.  SCALAR will be grown
or shrunk to the length actually read.  An OFFSET may be specified to
place the read data at some other place than the beginning of the
string.  This call is actually implemented in terms of stdio's fread(3)
call.  To get a true read(2) system call, see C<sysread>.

=item readdir DIRHANDLE

Returns the next directory entry for a directory opened by C<opendir>.
If used in list context, returns all the rest of the entries in the
directory.  If there are no more entries, returns an undefined value in
scalar context or a null list in list context.

If you're planning to filetest the return values out of a C<readdir>, you'd
better prepend the directory in question.  Otherwise, because we didn't
C<chdir> there, it would have been testing the wrong file.

    opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
    closedir DIR;

=item readline EXPR

Reads from the filehandle whose typeglob is contained in EXPR.  In scalar
context, each call reads and returns the next line, until end-of-file is
reached, whereupon the subsequent call returns undef.  In list context,
reads until end-of-file is reached and returns a list of lines.  Note that
the notion of "line" used here is however you may have defined it
with C<$/> or C<$INPUT_RECORD_SEPARATOR>).  See L<perlvar/"$/">.

When C<$/> is set to C<undef>, when readline() is in scalar
context (i.e. file slurp mode), and when an empty file is read, it
returns C<''> the first time, followed by C<undef> subsequently.

This is the internal function implementing the C<E<lt>EXPRE<gt>>
operator, but you can use it directly.  The C<E<lt>EXPRE<gt>>
operator is discussed in more detail in L<perlop/"I/O Operators">.

    $line = <STDIN>;
    $line = readline(*STDIN);           # same thing

=item readlink EXPR

=item readlink

Returns the value of a symbolic link, if symbolic links are
implemented.  If not, gives a fatal error.  If there is some system
error, returns the undefined value and sets C<$!> (errno).  If EXPR is
omitted, uses C<$_>.

=item readpipe EXPR

EXPR is executed as a system command.
The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially
multi-line) string.  In list context, returns a list of lines
(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
This is the internal function implementing the C<qx/EXPR/>
operator, but you can use it directly.  The C<qx/EXPR/>
operator is discussed in more detail in L<perlop/"I/O Operators">.

=item recv SOCKET,SCALAR,LENGTH,FLAGS

Receives a message on a socket.  Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.  SCALAR
will be grown or shrunk to the length actually read.  Takes the same
flags as the system call of the same name.  Returns the address of the
sender if SOCKET's protocol supports this; returns an empty string
otherwise.  If there's an error, returns the undefined value.  This call
is actually implemented in terms of recvfrom(2) system call.  See
L<perlipc/"UDP: Message Passing"> for examples.

=item redo LABEL

=item redo

The C<redo> command restarts the loop block without evaluating the
conditional again.  The C<continue> block, if any, is not executed.  If
the LABEL is omitted, the command refers to the innermost enclosing
loop.  This command is normally used by programs that want to lie to
themselves about what was just input:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
        while (s|({.*}.*){.*}|$1 |) {}
        s|{.*}| |;
        if (s|{.*| |) {
            $front = $_;
            while (<STDIN>) {
                if (/}/) {      # end of comment?
                    s|^|$front\{|;
                    redo LINE;
                }
            }
        }
        print;
    }

C<redo> cannot be used to retry a block which returns a value such as
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.

Note that a block by itself is semantically identical to a loop
that executes once.  Thus C<redo> inside such a block will effectively
turn it into a looping construct.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item ref EXPR

=item ref

Returns a true value if EXPR is a reference, false otherwise.  If EXPR
is not specified, C<$_> will be used.  The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:

    SCALAR
    ARRAY
    HASH
    CODE
    REF
    GLOB
    LVALUE

If the referenced object has been blessed into a package, then that package
name is returned instead.  You can think of C<ref> as a C<typeof> operator.

    if (ref($r) eq "HASH") {
        print "r is a reference to a hash.\n";
    }
    unless (ref($r)) {
        print "r is not a reference at all.\n";
    }
    if (UNIVERSAL::isa($r, "HASH")) {  # for subclassing
        print "r is a reference to something that isa hash.\n";
    } 

See also L<perlref>.

=item rename OLDNAME,NEWNAME

Changes the name of a file; an existing file NEWNAME will be
clobbered.  Returns true for success, false otherwise.

Behavior of this function varies wildly depending on your system
implementation.  For example, it will usually not work across file system
boundaries, even though the system I<mv> command sometimes compensates
for this.  Other restrictions include whether it works on directories,
open files, or pre-existing files.  Check L<perlport> and either the
rename(2) manpage or equivalent system documentation for details.

=item require EXPR

=item require

Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
supplied.  If EXPR is numeric, demands that the current version of Perl
(C<$]> or $PERL_VERSION) be equal or greater than EXPR.

Otherwise, demands that a library file be included if it hasn't already
been included.  The file is included via the do-FILE mechanism, which is
essentially just a variety of C<eval>.  Has semantics similar to the following
subroutine:

    sub require {
        my($filename) = @_;
        return 1 if $INC{$filename};
        my($realfilename,$result);
        ITER: {
            foreach $prefix (@INC) {
                $realfilename = "$prefix/$filename";
                if (-f $realfilename) {
                    $INC{$filename} = $realfilename;
                    $result = do $realfilename;
                    last ITER;
                }
            }
            die "Can't find $filename in \@INC";
        }
        delete $INC{$filename} if $@ || !$result;
        die $@ if $@;
        die "$filename did not return true value" unless $result;
        return $result;
    }

Note that the file will not be included twice under the same specified
name.  The file must return true as the last statement to indicate
successful execution of any initialization code, so it's customary to
end such a file with C<1;> unless you're sure it'll return true
otherwise.  But it's better just to put the C<1;>, in case you add more
statements.

If EXPR is a bareword, the require assumes a "F<.pm>" extension and
replaces "F<::>" with "F</>" in the filename for you,
to make it easy to load standard modules.  This form of loading of
modules does not risk altering your namespace.

In other words, if you try this:

        require Foo::Bar;    # a splendid bareword 

The require function will actually look for the "F<Foo/Bar.pm>" file in the 
directories specified in the C<@INC> array.

But if you try this:

        $class = 'Foo::Bar';
        require $class;      # $class is not a bareword
    #or
        require "Foo::Bar";  # not a bareword because of the ""

The require function will look for the "F<Foo::Bar>" file in the @INC array and 
will complain about not finding "F<Foo::Bar>" there.  In this case you can do:

        eval "require $class";

For a yet-more-powerful import facility, see L</use> and L<perlmod>.

=item reset EXPR

=item reset

Generally used in a C<continue> block at the end of a loop to clear
variables and reset C<??> searches so that they work again.  The
expression is interpreted as a list of single characters (hyphens
allowed for ranges).  All variables and arrays beginning with one of
those letters are reset to their pristine state.  If the expression is
omitted, one-match searches (C<?pattern?>) are reset to match again.  Resets
only variables or searches in the current package.  Always returns
1.  Examples:

    reset 'X';          # reset all X variables
    reset 'a-z';        # reset lower case variables
    reset;              # just reset ?one-time? searches

Resetting C<"A-Z"> is not recommended because you'll wipe out your
C<@ARGV> and C<@INC> arrays and your C<%ENV> hash.  Resets only package
variables--lexical variables are unaffected, but they clean themselves
up on scope exit anyway, so you'll probably want to use them instead.
See L</my>.

=item return EXPR

=item return

Returns from a subroutine, C<eval>, or C<do FILE> with the value 
given in EXPR.  Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see C<wantarray>).  If no EXPR
is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in a void context.

(Note that in the absence of a explicit C<return>, a subroutine, eval,
or do FILE will automatically return the value of the last expression
evaluated.)

=item reverse LIST

In list context, returns a list value consisting of the elements
of LIST in the opposite order.  In scalar context, concatenates the
elements of LIST and returns a string value with all characters
in the opposite order.

    print reverse <>;           # line tac, last line first

    undef $/;                   # for efficiency of <>
    print scalar reverse <>;    # character tac, last line tsrif

This operator is also handy for inverting a hash, although there are some
caveats.  If a value is duplicated in the original hash, only one of those
can be represented as a key in the inverted hash.  Also, this has to
unwind one hash and build a whole new one, which may take some time
on a large hash, such as from a DBM file.

    %by_name = reverse %by_address;     # Invert the hash

=item rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the
C<readdir> routine on DIRHANDLE.

=item rindex STR,SUBSTR,POSITION

=item rindex STR,SUBSTR

Works just like index() except that it returns the position of the LAST
occurrence of SUBSTR in STR.  If POSITION is specified, returns the
last occurrence at or before that position.

=item rmdir FILENAME

=item rmdir

Deletes the directory specified by FILENAME if that directory is empty.  If it
succeeds it returns true, otherwise it returns false and sets C<$!> (errno).  If
FILENAME is omitted, uses C<$_>.

=item s///

The substitution operator.  See L<perlop>.

=item scalar EXPR

Forces EXPR to be interpreted in scalar context and returns the value
of EXPR.

    @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to
be interpolated in list context because in practice, this is never
needed.  If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.

Because C<scalar> is unary operator, if you accidentally use for EXPR a
parenthesized list, this behaves as a scalar comma expression, evaluating
all but the last element in void context and returning the final element
evaluated in scalar context.  This is seldom what you want.

The following single statement:

        print uc(scalar(&foo,$bar)),$baz;

is the moral equivalent of these two:

        &foo;
        print(uc($bar),$baz);

See L<perlop> for more details on unary operators and the comma operator.

=item seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
FILEHANDLE may be an expression whose value gives the name of the
filehandle.  The values for WHENCE are C<0> to set the new position to
POSITION, C<1> to set it to the current position plus POSITION, and
C<2> to set it to EOF plus POSITION (typically negative).  For WHENCE
you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
(start of the file, current position, end of the file) from any of the
modules Fcntl, C<IO::Seekable>, or POSIX.  Returns C<1> upon success,
C<0> otherwise.

If you want to position file for C<sysread> or C<syswrite>, don't use
C<seek>--buffering makes its effect on the file's system position
unpredictable and non-portable.  Use C<sysseek> instead.

Due to the rules and rigors of ANSI C, on some systems you have to do a
seek whenever you switch between reading and writing.  Amongst other
things, this may have the effect of calling stdio's clearerr(3).
A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:

    seek(TEST,0,1);

This is also useful for applications emulating C<tail -f>.  Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
seek() to reset things.  The C<seek> doesn't change the current position,
but it I<does> clear the end-of-file condition on the handle, so that the
next C<E<lt>FILEE<gt>> makes Perl try again to read something.  We hope.

If that doesn't work (some stdios are particularly cantankerous), then
you may need something more like this:

    for (;;) {
        for ($curpos = tell(FILE); $_ = <FILE>;
             $curpos = tell(FILE)) {
            # search for some stuff and put it into files
        }
        sleep($for_a_while);
        seek(FILE, $curpos, 0);
    }

=item seekdir DIRHANDLE,POS

Sets the current position for the C<readdir> routine on DIRHANDLE.  POS
must be a value returned by C<telldir>.  Has the same caveats about
possible directory compaction as the corresponding system library
routine.

=item select FILEHANDLE

=item select

Returns the currently selected filehandle.  Sets the current default
filehandle for output, if FILEHANDLE is supplied.  This has two
effects: first, a C<write> or a C<print> without a filehandle will
default to this FILEHANDLE.  Second, references to variables related to
output will refer to this output channel.  For example, if you have to
set the top of form format for more than one output channel, you might
do the following:

    select(REPORT1);
    $^ = 'report1_top';
    select(REPORT2);
    $^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the
actual filehandle.  Thus:

    $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with
methods, preferring to write the last example as:

    use IO::Handle;
    STDERR->autoflush(1);

=item select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) system call with the bit masks specified, which
can be constructed using C<fileno> and C<vec>, along these lines:

    $rin = $win = $ein = '';
    vec($rin,fileno(STDIN),1) = 1;
    vec($win,fileno(STDOUT),1) = 1;
    $ein = $rin | $win;

If you want to select on many filehandles you might wish to write a
subroutine:

    sub fhbits {
        my(@fhlist) = split(' ',$_[0]);
        my($bits);
        for (@fhlist) {
            vec($bits,fileno($_),1) = 1;
        }
        $bits;
    }
    $rin = fhbits('STDIN TTY SOCK');

The usual idiom is:

    ($nfound,$timeleft) =
      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready just do this

    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

Most systems do not bother to return anything useful in $timeleft, so
calling select() in scalar context just returns $nfound.

Any of the bit masks can also be undef.  The timeout, if specified, is
in seconds, which may be fractional.  Note: not all implementations are
capable of returning the$timeleft.  If not, they always return
$timeleft equal to the supplied $timeout.

You can effect a sleep of 250 milliseconds this way:

    select(undef, undef, undef, 0.25);

B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
then only on POSIX systems.  You have to use C<sysread> instead.

=item semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function C<semctl>.  You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions.  If CMD is IPC_STAT or
GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array.  Returns like C<ioctl>: the
undefined value for error, "C<0 but true>" for zero, or the actual return
value otherwise.  See also C<IPC::SysV> and C<IPC::Semaphore> documentation.

=item semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget.  Returns the semaphore id, or
the undefined value if there is an error.  See also C<IPC::SysV> and
C<IPC::SysV::Semaphore> documentation.

=item semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations
such as signaling and waiting.  OPSTRING must be a packed array of
semop structures.  Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>.  The number of semaphore
operations is implied by the length of OPSTRING.  Returns true if
successful, or false if there is an error.  As an example, the
following code waits on semaphore $semnum of semaphore id $semid:

    $semop = pack("sss", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace C<-1> with C<1>.  See also C<IPC::SysV>
and C<IPC::SysV::Semaphore> documentation.

=item send SOCKET,MSG,FLAGS,TO

=item send SOCKET,MSG,FLAGS

Sends a message on a socket.  Takes the same flags as the system call
of the same name.  On unconnected sockets you must specify a
destination to send TO, in which case it does a C C<sendto>.  Returns
the number of characters sent, or the undefined value if there is an
error.  The C system call sendmsg(2) is currently unimplemented.
See L<perlipc/"UDP: Message Passing"> for examples.

=item setpgrp PID,PGRP

Sets the current process group for the specified PID, C<0> for the current
process.  Will produce a fatal error if used on a machine that doesn't
implement POSIX setpgid(2) or BSD setpgrp(2).  If the arguments are omitted,
it defaults to C<0,0>.  Note that the BSD 4.2 version of C<setpgrp> does not
accept any arguments, so only C<setpgrp(0,0)> is portable.  See also
C<POSIX::setsid()>.

=item setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user.
(See setpriority(2).)  Will produce a fatal error if used on a machine
that doesn't implement setpriority(2).

=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested.  Returns undefined if there is an
error.  OPTVAL may be specified as C<undef> if you don't want to pass an
argument.

=item shift ARRAY

=item shift

Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down.  If there are no elements in the
array, returns the undefined value.  If ARRAY is omitted, shifts the
C<@_> array within the lexical scope of subroutines and formats, and the
C<@ARGV> array at file scopes or within the lexical scopes established by
the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<STOP {}>, and C<END {}>
constructs.

See also C<unshift>, C<push>, and C<pop>.  C<Shift()> and C<unshift> do the
same thing to the left end of an array that C<pop> and C<push> do to the
right end.

=item shmctl ID,CMD,ARG

Calls the System V IPC function shmctl.  You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions.  If CMD is C<IPC_STAT>,
then ARG must be a variable which will hold the returned C<shmid_ds>
structure.  Returns like ioctl: the undefined value for error, "C<0> but
true" for zero, or the actual return value otherwise.
See also C<IPC::SysV> documentation.

=item shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget.  Returns the shared memory
segment id, or the undefined value if there is an error.
See also C<IPC::SysV> documentation.

=item shmread ID,VAR,POS,SIZE

=item shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
detaching from it.  When reading, VAR must be a variable that will
hold the data read.  When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes.  Return true if successful, or false if there is an error.
See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
from CPAN.

=item shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the system call of the same name.

    shutdown(SOCKET, 0);    # I/we have stopped reading data
    shutdown(SOCKET, 1);    # I/we have stopped writing data
    shutdown(SOCKET, 2);    # I/we have stopped using this socket

This is useful with sockets when you want to tell the other
side you're done writing but not done reading, or vice versa.
It's also a more insistent form of close because it also 
disables the file descriptor in any forked copies in other
processes.

=item sin EXPR

=item sin

Returns the sine of EXPR (expressed in radians).  If EXPR is omitted,
returns sine of C<$_>.

For the inverse sine operation, you may use the C<POSIX::asin>
function, or use this relation:

    sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

=item sleep EXPR

=item sleep

Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted if the process receives a signal such as C<SIGALRM>.
Returns the number of seconds actually slept.  You probably cannot
mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
using C<alarm>.

On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds.  Most modern systems
always sleep the full amount.  They may appear to sleep longer than that,
however, because your process might not be scheduled right away in a
busy multitasking system.

For delays of finer granularity than one second, you may use Perl's
C<syscall> interface to access setitimer(2) if your system supports
it, or else see L</select> above.  The Time::HiRes module from CPAN
may also help.

See also the POSIX module's C<sigpause> function.

=item socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle
SOCKET.  DOMAIN, TYPE, and PROTOCOL are specified the same as for
the system call of the same name.  You should C<use Socket> first
to get the proper definitions imported.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the
specified type.  DOMAIN, TYPE, and PROTOCOL are specified the same as
for the system call of the same name.  If unimplemented, yields a fatal
error.  Returns true if successful.

Some systems defined C<pipe> in terms of C<socketpair>, in which a call
to C<pipe(Rdr, Wtr)> is essentially:

    use Socket;
    socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
    shutdown(Rdr, 1);        # no more writing for reader
    shutdown(Wtr, 0);        # no more reading for writer

See L<perlipc> for an example of socketpair use.

=item sort SUBNAME LIST

=item sort BLOCK LIST

=item sort LIST

Sorts the LIST and returns the sorted list value.  If SUBNAME or BLOCK
is omitted, C<sort>s in standard string comparison order.  If SUBNAME is
specified, it gives the name of a subroutine that returns an integer
less than, equal to, or greater than C<0>, depending on how the elements
of the list are to be ordered.  (The C<E<lt>=E<gt>> and C<cmp>
operators are extremely useful in such routines.)  SUBNAME may be a
scalar variable name (unsubscripted), in which case the value provides
the name of (or a reference to) the actual subroutine to use.  In place
of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.

If the subroutine's prototype is C<($$)>, the elements to be compared
are passed by reference in C<@_>, as for a normal subroutine.  If not,
the normal calling code for subroutines is bypassed in the interests of
efficiency, and the elements to be compared are passed into the subroutine
as the package global variables $a and $b (see example below).  Note that
in the latter case, it is usually counter-productive to declare $a and
$b as lexicals.

In either case, the subroutine may not be recursive.  The values to be
compared are always passed by reference, so don't modify them.

You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in L<perlsyn> or with C<goto>.

When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
current collation locale.  See L<perllocale>.

Examples:

    # sort lexically
    @articles = sort @files;

    # same thing, but with explicit sort routine
    @articles = sort {$a cmp $b} @files;

    # now case-insensitively
    @articles = sort {uc($a) cmp uc($b)} @files;

    # same thing in reversed order
    @articles = sort {$b cmp $a} @files;

    # sort numerically ascending
    @articles = sort {$a <=> $b} @files;

    # sort numerically descending
    @articles = sort {$b <=> $a} @files;

    # this sorts the %age hash by value instead of key
    # using an in-line function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

    # sort using explicit subroutine name
    sub byage {
        $age{$a} <=> $age{$b};  # presuming numeric
    }
    @sortedclass = sort byage @class;

    sub backwards { $b cmp $a }
    @harry  = qw(dog cat x Cain Abel);
    @george = qw(gone chased yz Punished Axed);
    print sort @harry;
            # prints AbelCaincatdogx
    print sort backwards @harry;
            # prints xdogcatCainAbel
    print sort @george, 'to', @harry;
            # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using
    # the first integer after the first = sign, or the
    # whole record case-insensitively otherwise

    @new = sort {
        ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                            ||
                    uc($a)  cmp  uc($b)
    } @old;

    # same thing, but much more efficiently;
    # we'll build auxiliary indices instead
    # for speed
    @nums = @caps = ();
    for (@old) {
        push @nums, /=(\d+)/;
        push @caps, uc($_);
    }

    @new = @old[ sort {
                        $nums[$b] <=> $nums[$a]
                                 ||
                        $caps[$a] cmp $caps[$b]
                       } 0..$#old
               ];

    # same thing, but without any temps
    @new = map { $_->[0] }
           sort { $b->[1] <=> $a->[1]
                           ||
                  $a->[2] cmp $b->[2]
           } map { [$_, /=(\d+)/, uc($_)] } @old;
    
    # using a prototype allows you to use any comparison subroutine
    # as a sort subroutine (including other package's subroutines)
    package other;
    sub backwards ($$) { $_[1] cmp $_[0]; }     # $a and $b are not set here

    package main;
    @new = sort other::backwards @old;

If you're using strict, you I<must not> declare $a
and $b as lexicals.  They are package globals.  That means
if you're in the C<main> package, it's

    @articles = sort {$main::b <=> $main::a} @files;

or just

    @articles = sort {$::b <=> $::a} @files;

but if you're in the C<FooPack> package, it's

    @articles = sort {$FooPack::b <=> $FooPack::a} @files;

The comparison function is required to behave.  If it returns
inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
sometimes saying the opposite, for example) the results are not
well-defined.

=item splice ARRAY,OFFSET,LENGTH,LIST

=item splice ARRAY,OFFSET,LENGTH

=item splice ARRAY,OFFSET

Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any.  In list context,
returns the elements removed from the array.  In scalar context,
returns the last element removed, or C<undef> if no elements are
removed.  The array grows or shrinks as necessary.
If OFFSET is negative then it starts that far from the end of the array.
If LENGTH is omitted, removes everything from OFFSET onward.
If LENGTH is negative, leave that many elements off the end of the array.
The following equivalences hold (assuming C<$[ == 0>):

    push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
    pop(@a)             splice(@a,-1)
    shift(@a)           splice(@a,0,1)
    unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
    $a[$x] = $y         splice(@a,$x,1,$y)

Example, assuming array lengths are passed before arrays:

    sub aeq {   # compare two list values
        my(@a) = splice(@_,0,shift);
        my(@b) = splice(@_,0,shift);
        return 0 unless @a == @b;       # same len?
        while (@a) {
            return 0 if pop(@a) ne pop(@b);
        }
        return 1;
    }
    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

=item split /PATTERN/,EXPR,LIMIT

=item split /PATTERN/,EXPR

=item split /PATTERN/

=item split

Splits a string into a list of strings and returns that list.  By default,
empty leading fields are preserved, and empty trailing ones are deleted.

If not in list context, returns the number of fields found and splits into
the C<@_> array.  (In list context, you can force the split into C<@_> by
using C<??> as the pattern delimiters, but it still returns the list
value.)  The use of implicit split to C<@_> is deprecated, however, because
it clobbers your subroutine arguments.

If EXPR is omitted, splits the C<$_> string.  If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace).  Anything
matching PATTERN is taken to be a delimiter separating the fields.  (Note
that the delimiter may be longer than one character.)

If LIMIT is specified and positive, splits into no more than that
many fields (though it may split into fewer).  If LIMIT is unspecified
or zero, trailing null fields are stripped (which potential users
of C<pop> would do well to remember).  If LIMIT is negative, it is
treated as if an arbitrarily large LIMIT had been specified.

A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way.  For example:

    print join(':', split(/ */, 'hi there'));

produces the output 'h:i:t:h:e:r:e'.

The LIMIT parameter can be used to split a line partially

    ($login, $passwd, $remainder) = split(/:/, $_, 3);

When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
one larger than the number of variables in the list, to avoid
unnecessary work.  For the list above LIMIT would have been 4 by
default.  In time critical applications it behooves you not to split
into more fields than you really need.

If the PATTERN contains parentheses, additional list elements are
created from each matching substring in the delimiter.

    split(/([,-])/, "1-10,20", 3);

produces the list value

    (1, '-', 10, ',', 20)

If you had the entire header of a normal Unix email message in $header,
you could split it up into fields and their values this way:

    $header =~ s/\n\s+/ /g;  # fix continuation lines
    %hdrs   =  (UNIX_FROM => split /^(\S*?):\s*/m, $header);

The pattern C</PATTERN/> may be replaced with an expression to specify
patterns that vary at runtime.  (To do runtime compilation only once,
use C</$variable/o>.)

As a special case, specifying a PATTERN of space (C<' '>) will split on
white space just as C<split> with no arguments does.  Thus, C<split(' ')> can
be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
will give you as many null initial fields as there are leading spaces.
A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
whitespace produces a null first field.  A C<split> with no arguments
really does a C<split(' ', $_)> internally.

Example:

    open(PASSWD, '/etc/passwd');
    while (<PASSWD>) {
        ($login, $passwd, $uid, $gid,
         $gcos, $home, $shell) = split(/:/);
        #...
    }

(Note that $shell above will still have a newline on it.  See L</chop>,
L</chomp>, and L</join>.)

=item sprintf FORMAT, LIST

Returns a string formatted by the usual C<printf> conventions of the
C library function C<sprintf>.  See L<sprintf(3)> or L<printf(3)>
on your system for an explanation of the general principles.

Perl does its own C<sprintf> formatting--it emulates the C
function C<sprintf>, but it doesn't use it (except for floating-point
numbers, and even then only the standard modifiers are allowed).  As a
result, any non-standard extensions in your local C<sprintf> are not
available from Perl.

Perl's C<sprintf> permits the following universally-known conversions:

   %%   a percent sign
   %c   a character with the given number
   %s   a string
   %d   a signed integer, in decimal
   %u   an unsigned integer, in decimal
   %o   an unsigned integer, in octal
   %x   an unsigned integer, in hexadecimal
   %e   a floating-point number, in scientific notation
   %f   a floating-point number, in fixed decimal notation
   %g   a floating-point number, in %e or %f notation

In addition, Perl permits the following widely-supported conversions:

   %X   like %x, but using upper-case letters
   %E   like %e, but using an upper-case "E"
   %G   like %g, but with an upper-case "E" (if applicable)
   %b   an unsigned integer, in binary
   %p   a pointer (outputs the Perl value's address in hexadecimal)
   %n   special: *stores* the number of characters output so far
        into the next variable in the parameter list 

Finally, for backward (and we do mean "backward") compatibility, Perl
permits these unnecessary but widely-supported conversions:

   %i   a synonym for %d
   %D   a synonym for %ld
   %U   a synonym for %lu
   %O   a synonym for %lo
   %F   a synonym for %f

Perl permits the following universally-known flags between the C<%>
and the conversion letter:

   space   prefix positive number with a space
   +       prefix positive number with a plus sign
   -       left-justify within the field
   0       use zeros, not spaces, to right-justify
   #       prefix non-zero octal with "0", non-zero hex with "0x"
   number  minimum field width
   .number "precision": digits after decimal point for
           floating-point, max length for string, minimum length
           for integer
   l       interpret integer as C type "long" or "unsigned long"
   h       interpret integer as C type "short" or "unsigned short"
           If no flags, interpret integer as C type "int" or "unsigned"

There is also one Perl-specific flag:

   V       interpret integer as Perl's standard integer type

Where a number would appear in the flags, an asterisk (C<*>) may be
used instead, in which case Perl uses the next item in the parameter
list as the given number (that is, as the field width or precision).
If a field width obtained through C<*> is negative, it has the same
effect as the C<-> flag: left-justification.

If C<use locale> is in effect, the character used for the decimal
point in formatted real numbers is affected by the LC_NUMERIC locale.
See L<perllocale>.

If Perl understands "quads" (64-bit integers) (this requires
either that the platform natively supports quads or that Perl
has been specifically compiled to support quads), the characters

        d u o x X b i D U O

print quads, and they may optionally be preceded by

        ll L q

For example

        %lld %16LX %qo

You can find out whether your Perl supports quads via L<Config>:

        use Config;
        ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
                print "quads\n";

If Perl understands "long doubles" (this requires that the platform
supports long doubles), the flags

        e f g E F G

may optionally be preceded by

        ll L

For example

        %llf %Lg

You can find out whether your Perl supports long doubles via L<Config>:

        use Config;
        $Config{d_longdbl} eq 'define' && print "long doubles\n";

=item sqrt EXPR

=item sqrt

Return the square root of EXPR.  If EXPR is omitted, returns square
root of C<$_>.  Only works on non-negative operands, unless you've
loaded the standard Math::Complex module.

    use Math::Complex;
    print sqrt(-2);    # prints 1.4142135623731i

=item srand EXPR

=item srand

Sets the random number seed for the C<rand> operator.  If EXPR is
omitted, uses a semi-random value supplied by the kernel (if it supports
the F</dev/urandom> device) or based on the current time and process
ID, among other things.  In versions of Perl prior to 5.004 the default
seed was just the current C<time>.  This isn't a particularly good seed,
so many old programs supply their own seed value (often C<time ^ $$> or
C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.

In fact, it's usually not necessary to call C<srand> at all, because if
it is not called explicitly, it is called implicitly at the first use of
the C<rand> operator.  However, this was not the case in version of Perl
before 5.004, so if your script will run under older Perl versions, it
should call C<srand>.

Note that you need something much more random than the default seed for
cryptographic purposes.  Checksumming the compressed output of one or more
rapidly changing operating system status programs is the usual method.  For
example:

    srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);

If you're particularly concerned with this, see the C<Math::TrulyRandom>
module in CPAN.

Do I<not> call C<srand> multiple times in your program unless you know
exactly what you're doing and why you're doing it.  The point of the
function is to "seed" the C<rand> function so that C<rand> can produce
a different sequence each time you run your program.  Just do it once at the
top of your program, or you I<won't> get random numbers out of C<rand>!

Frequently called programs (like CGI scripts) that simply use

    time ^ $$

for a seed can fall prey to the mathematical property that

    a^b == (a+1)^(b+1)

one-third of the time.  So don't do that.

=item stat FILEHANDLE

=item stat EXPR

=item stat

Returns a 13-element list giving the status info for a file, either
the file opened via FILEHANDLE, or named by EXPR.  If EXPR is omitted,
it stats C<$_>.  Returns a null list if the stat fails.  Typically used
as follows:

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types.  Here are the
meaning of the fields:

  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions)
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) since the epoch
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If stat is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
last stat or filetest are returned.  Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }

(This works on machines only for which the device number is negative under NFS.)

Because the mode contains both the file type and its permissions, you
should mask off the file type portion and (s)printf using a C<"%o"> 
if you want to see the real permissions.

    $mode = (stat($filename))[2];
    printf "Permissions are %04o\n", $mode & 07777;

In scalar context, C<stat> returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle C<_>.

The File::stat module provides a convenient, by-name access mechanism:

    use File::stat;
    $sb = stat($filename);
    printf "File is %s, size is %s, perm %04o, mtime %s\n", 
        $filename, $sb->size, $sb->mode & 07777,
        scalar localtime $sb->mtime;

=item study SCALAR

=item study

Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
run times with and without it to see which runs faster.  Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most.  You may have only
one C<study> active at a time--if you study a different scalar the first
is "unstudied".  (The way C<study> works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the C<'k'> characters are.  From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text.  Only those places
that contain this "rarest" character are examined.)

For example, here is a loop that inserts index producing entries
before any line containing a certain pattern:

    while (<>) {
        study;
        print ".IX foo\n"       if /\bfoo\b/;
        print ".IX bar\n"       if /\bbar\b/;
        print ".IX blurfl\n"    if /\bblurfl\b/;
        # ...
        print;
    }

In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
will be looked at, because C<f> is rarer than C<o>.  In general, this is
a big win except in pathological cases.  The only question is whether
it saves you more time than it took to build the linked list in the
first place.

Note that if you have to look for strings that you don't know till
runtime, you can build an entire loop as a string and C<eval> that to
avoid recompiling all your patterns all the time.  Together with
undefining C<$/> to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1).  The following
scans a list of files (C<@files>) for a list of words (C<@words>), and prints
out the names of those files that contain a match:

    $search = 'while (<>) { study;';
    foreach $word (@words) {
        $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;               # this screams
    $/ = "\n";          # put back to normal input delimiter
    foreach $file (sort keys(%seen)) {
        print $file, "\n";
    }

=item sub BLOCK

=item sub NAME

=item sub NAME BLOCK

This is subroutine definition, not a real function I<per se>.  With just a
NAME (and possibly prototypes or attributes), it's just a forward declaration.
Without a NAME, it's an anonymous function declaration, and does actually
return a value: the CODE ref of the closure you just created.  See L<perlsub>
and L<perlref> for details.

=item substr EXPR,OFFSET,LENGTH,REPLACEMENT

=item substr EXPR,OFFSET,LENGTH

=item substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it.  First character is at
offset C<0>, or whatever you've set C<$[> to (but don't do that).
If OFFSET is negative (or more precisely, less than C<$[>), starts
that far from the end of the string.  If LENGTH is omitted, returns
everything to the end of the string.  If LENGTH is negative, leaves that
many characters off the end of the string.

You can use the substr() function as an lvalue, in which case EXPR
must itself be an lvalue.  If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it.  To keep the string the same
length you may need to pad or chop your value using C<sprintf>.

If OFFSET and LENGTH specify a substring that is partly outside the
string, only the part within the string is returned.  If the substring
is beyond either end of the string, substr() returns the undefined
value and produces a warning.  When used as an lvalue, specifying a
substring that is entirely outside the string is a fatal error.
Here's an example showing the behavior for boundary cases:

    my $name = 'fred';
    substr($name, 4) = 'dy';            # $name is now 'freddy'
    my $null = substr $name, 6, 2;      # returns '' (no warning)
    my $oops = substr $name, 7;         # returns undef, with warning
    substr($name, 7) = 'gap';           # fatal error

An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument.  This allows you to replace
parts of the EXPR and return what was there before in one operation,
just as you can with splice().

=item symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename.
Returns C<1> for success, C<0> otherwise.  On systems that don't support
symbolic links, produces a fatal error at run time.  To check for that,
use eval:

    $symlink_exists = eval { symlink("",""); 1 };

=item syscall LIST

Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call.  If
unimplemented, produces a fatal error.  The arguments are interpreted
as follows: if a given argument is numeric, the argument is passed as
an int.  If not, the pointer to the string value is passed.  You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string.  You can't use a
string literal (or other read-only string) as an argument to C<syscall>
because Perl has to assume that any string pointer might be written
through.  If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add C<0> to them to force them to look
like numbers.  This emulates the C<syswrite> function (or vice versa):

    require 'syscall.ph';               # may need to run h2ph
    $s = "hi there\n";
    syscall(&SYS_write, fileno(STDOUT), $s, length $s);

Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.

Syscall returns whatever value returned by the system call it calls.
If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
Note that some system calls can legitimately return C<-1>.  The proper
way to handle such calls is to assign C<$!=0;> before the call and
check the value of C<$!> if syscall returns C<-1>.

There's a problem with C<syscall(&SYS_pipe)>: it returns the file
number of the read end of the pipe it creates.  There is no way
to retrieve the file number of the other end.  You can avoid this 
problem by using C<pipe> instead.

=item sysopen FILEHANDLE,FILENAME,MODE

=item sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it
with FILEHANDLE.  If FILEHANDLE is an expression, its value is used as
the name of the real filehandle wanted.  This function calls the
underlying operating system's C<open> function with the parameters
FILENAME, MODE, PERMS.

The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module C<Fcntl>.
See the documentation of your operating system's C<open> to see which
values and flag bits are available.  You may combine several flags
using the C<|>-operator.

Some of the most common values are C<O_RDONLY> for opening the file in
read-only mode, C<O_WRONLY> for opening the file in write-only mode,
and C<O_RDWR> for opening the file in read-write mode, and.

For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only, and two
means read/write.  We know that these values do I<not> work under
OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
use them in new code.

If the file named by FILENAME does not exist and the C<open> call creates
it (typically because MODE includes the C<O_CREAT> flag), then the value of
PERMS specifies the permissions of the newly created file.  If you omit
the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
These permission values need to be in octal, and are modified by your
process's current C<umask>.

In many systems the C<O_EXCL> flag is available for opening files in
exclusive mode.  This is B<not> locking: exclusiveness means here that
if the file already exists, sysopen() fails.  The C<O_EXCL> wins
C<O_TRUNC>.

Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.

You should seldom if ever use C<0644> as argument to C<sysopen>, because
that takes away the user's option to have a more permissive umask.
Better to omit it.  See the perlfunc(1) entry on C<umask> for more
on this.

Note that C<sysopen> depends on the fdopen() C library function.
On many UNIX systems, fdopen() is known to fail when file descriptors
exceed a certain value, typically 255. If you need more file
descriptors than that, consider rebuilding Perl to use the C<sfio>
library, or perhaps using the POSIX::open() function.

See L<perlopentut> for a kinder, gentler explanation of opening files.

=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

=item sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the system call read(2).  It bypasses stdio,
so mixing this with other kinds of reads, C<print>, C<write>,
C<seek>, C<tell>, or C<eof> can cause confusion because stdio
usually buffers data.  Returns the number of bytes actually read, C<0>
at end of file, or undef if there was an error.  SCALAR will be grown or
shrunk so that the last byte actually read is the last byte of the
scalar after the read.

An OFFSET may be specified to place the read data at some place in the
string other than the beginning.  A negative OFFSET specifies
placement at that many bytes counting backwards from the end of the
string.  A positive OFFSET greater than the length of SCALAR results
in the string being padded to the required size with C<"\0"> bytes before
the result of the read is appended.

There is no syseof() function, which is ok, since eof() doesn't work
very well on device files (like ttys) anyway.  Use sysread() and check
for a return value for 0 to decide whether you're done.

=item sysseek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's system position using the system call lseek(2).  It
bypasses stdio, so mixing this with reads (other than C<sysread>),
C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
FILEHANDLE may be an expression whose value gives the name of the
filehandle.  The values for WHENCE are C<0> to set the new position to
POSITION, C<1> to set the it to the current position plus POSITION,
and C<2> to set it to EOF plus POSITION (typically negative).  For
WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
C<SEEK_END> (start of the file, current position, end of the file)
from any of the modules Fcntl, C<IO::Seekable>, or POSIX.

Returns the new position, or the undefined value on failure.  A position
of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
true on success and false on failure, yet you can still easily determine
the new position.

=item system LIST

=item system PROGRAM LIST

Does exactly the same thing as C<exec LIST>, except that a fork is
done first, and the parent process waits for the child process to
complete.  Note that argument processing varies depending on the
number of arguments.  If there is more than one argument in LIST,
or if LIST is an array with more than one value, starts the program
given by the first element of the list with arguments given by the
rest of the list.  If there is only one scalar argument, the argument
is checked for shell metacharacters, and if there are any, the
entire argument is passed to the system's command shell for parsing
(this is C</bin/sh -c> on Unix platforms, but varies on other
platforms).  If there are no shell metacharacters in the argument,
it is split into words and passed directly to C<execvp>, which is
more efficient.

All files opened for output are flushed before attempting the exec().

The return value is the exit status of the program as
returned by the C<wait> call.  To get the actual exit value divide by
256.  See also L</exec>.  This is I<not> what you want to use to capture
the output from a command, for that you should use merely backticks or
C<qx//>, as described in L<perlop/"`STRING`">.  Return value of -1
indicates a failure to start the program (inspect $! for the reason).

Like C<exec>, C<system> allows you to lie to a program about its name if
you use the C<system PROGRAM LIST> syntax.  Again, see L</exec>.

Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
program they're running doesn't actually interrupt your program.

    @args = ("command", "arg1", "arg2");
    system(@args) == 0
         or die "system @args failed: $?"

You can check all the failure possibilities by inspecting
C<$?> like this:

    $exit_value  = $? >> 8;
    $signal_num  = $? & 127;
    $dumped_core = $? & 128;

When the arguments get executed via the system shell, results
and return codes will be subject to its quirks and capabilities.
See L<perlop/"`STRING`"> and L</exec> for details.

=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

=item syswrite FILEHANDLE,SCALAR,LENGTH

=item syswrite FILEHANDLE,SCALAR

Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using the system call write(2).  If LENGTH
is not specified, writes whole SCALAR.  It bypasses stdio, so mixing
this with reads (other than C<sysread())>, C<print>, C<write>,
C<seek>, C<tell>, or C<eof> may cause confusion because stdio
usually buffers data.  Returns the number of bytes actually written,
or C<undef> if there was an error.  If the LENGTH is greater than
the available data in the SCALAR after the OFFSET, only as much
data as is available will be written.

An OFFSET may be specified to write the data from some part of the
string other than the beginning.  A negative OFFSET specifies writing
that many bytes counting backwards from the end of the string.  In the
case the SCALAR is empty you can use OFFSET but only zero offset.

=item tell FILEHANDLE

=item tell

Returns the current position for FILEHANDLE.  FILEHANDLE may be an
expression whose value gives the name of the actual filehandle.  If
FILEHANDLE is omitted, assumes the file last read.  

There is no C<systell> function.  Use C<sysseek(FH, 0, 1)> for that.

=item telldir DIRHANDLE

Returns the current position of the C<readdir> routines on DIRHANDLE.
Value may be given to C<seekdir> to access a particular location in a
directory.  Has the same caveats about possible directory compaction as
the corresponding system library routine.

=item tie VARIABLE,CLASSNAME,LIST

This function binds a variable to a package class that will provide the
implementation for the variable.  VARIABLE is the name of the variable
to be enchanted.  CLASSNAME is the name of a class implementing objects
of correct type.  Any additional arguments are passed to the C<new>
method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
or C<TIEHASH>).  Typically these are arguments such as might be passed
to the C<dbm_open()> function of C.  The object returned by the C<new>
method is also returned by the C<tie> function, which would be useful
if you want to access other methods in CLASSNAME.

Note that functions such as C<keys> and C<values> may return huge lists
when used on large objects, like DBM files.  You may prefer to use the
C<each> function to iterate over such.  Example:

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

A class implementing a hash should have the following methods:

    TIEHASH classname, LIST
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    CLEAR this
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey
    DESTROY this

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    FETCH this, key
    STORE this, key, value
    FETCHSIZE this
    STORESIZE this, count
    CLEAR this
    PUSH this, LIST
    POP this
    SHIFT this
    UNSHIFT this, LIST
    SPLICE this, offset, length, LIST
    EXTEND this, count
    DESTROY this

A class implementing a file handle should have the following methods:

    TIEHANDLE classname, LIST
    READ this, scalar, length, offset
    READLINE this
    GETC this
    WRITE this, scalar, length, offset
    PRINT this, LIST
    PRINTF this, format, LIST
    CLOSE this
    DESTROY this

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    FETCH this,
    STORE this, value
    DESTROY this

Not all methods indicated above need be implemented.  See L<perltie>,
L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.

Unlike C<dbmopen>, the C<tie> function will not use or require a module
for you--you need to do that explicitly yourself.  See L<DB_File>
or the F<Config> module for interesting C<tie> implementations.

For further details see L<perltie>, L<"tied VARIABLE">.

=item tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value
that was originally returned by the C<tie> call that bound the variable
to a package.)  Returns the undefined value if VARIABLE isn't tied to a
package.

=item time

Returns the number of non-leap seconds since whatever time the system
considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
and 00:00:00 UTC, January 1, 1970 for most other systems).
Suitable for feeding to C<gmtime> and C<localtime>.

For measuring time in better granularity than one second,
you may use either the Time::HiRes module from CPAN, or
if you have gettimeofday(2), you may be able to use the
C<syscall> interface of Perl, see L<perlfaq8> for details.

=item times

Returns a four-element list giving the user and system times, in
seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;

=item tr///

The transliteration operator.  Same as C<y///>.  See L<perlop>.

=item truncate FILEHANDLE,LENGTH

=item truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length.  Produces a fatal error if truncate isn't implemented
on your system.  Returns true if successful, the undefined value
otherwise.

=item uc EXPR

=item uc

Returns an uppercased version of EXPR.  This is the internal function
implementing the C<\U> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.
Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings.  (It
does not attempt to do titlecase mapping on initial letters.  See C<ucfirst> for that.)

If EXPR is omitted, uses C<$_>.

=item ucfirst EXPR

=item ucfirst

Returns the value of EXPR with the first character
in uppercase (titlecase in Unicode).  This is
the internal function implementing the C<\u> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>
and L<utf8>.

If EXPR is omitted, uses C<$_>.

=item umask EXPR

=item umask

Sets the umask for the process to EXPR and returns the previous value.
If EXPR is omitted, merely returns the current umask.

The Unix permission C<rwxr-x---> is represented as three sets of three
bits, or three octal digits: C<0750> (the leading 0 indicates octal
and isn't one of the digits).  The C<umask> value is such a number
representing disabled permissions bits.  The permission (or "mode")
values you pass C<mkdir> or C<sysopen> are modified by your umask, so
even if you tell C<sysopen> to create a file with permissions C<0777>,
if your umask is C<0022> then the file will actually be created with
permissions C<0755>.  If your C<umask> were C<0027> (group can't
write; others can't read, write, or execute), then passing
C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
027> is C<0640>).

Here's some advice: supply a creation mode of C<0666> for regular
files (in C<sysopen>) and one of C<0777> for directories (in
C<mkdir>) and executable files.  This gives users the freedom of
choice: if they want protected files, they might choose process umasks
of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
Programs should rarely if ever make policy decisions better left to
the user.  The exception to this is when writing files that should be
kept private: mail files, web browser cookies, I<.rhosts> files, and
so on.

If umask(2) is not implemented on your system and you are trying to
restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
fatal error at run time.  If umask(2) is not implemented and you are
not trying to restrict access for yourself, returns C<undef>.

Remember that a umask is a number, usually given in octal; it is I<not> a
string of octal digits.  See also L</oct>, if all you have is a string.

=item undef EXPR

=item undef

Undefines the value of EXPR, which must be an lvalue.  Use only on a
scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
(using C<&>), or a typeglob (using <*>).  (Saying C<undef $hash{$key}>
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that; see L<delete>.)  Always returns the
undefined value.  You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could, for
instance, return from a subroutine, assign to a variable or pass as a
parameter.  Examples:

    undef $foo;
    undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
    undef @ary;
    undef %hash;
    undef &mysub;
    undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
    return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
    select undef, undef, undef, 0.25;
    ($a, $b, undef, $c) = &foo;       # Ignore third value returned

Note that this is a unary operator, not a list operator.

=item unlink LIST

=item unlink

Deletes a list of files.  Returns the number of files successfully
deleted.

    $cnt = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink <*.bak>;

Note: C<unlink> will not delete directories unless you are superuser and
the B<-U> flag is supplied to Perl.  Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
filesystem.  Use C<rmdir> instead.

If LIST is omitted, uses C<$_>.

=item unpack TEMPLATE,EXPR

C<unpack> does the reverse of C<pack>: it takes a string
and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)

The string is broken into chunks described by the TEMPLATE.  Each chunk
is converted separately to a value.  Typically, either the string is a result
of C<pack>, or the bytes of the string represent a C structure of some
kind.

The TEMPLATE has the same format as in the C<pack> function.
Here's a subroutine that does substring:

    sub substr {
        my($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition to fields allowed in pack(), you may prefix a field with
a %E<lt>numberE<gt> to indicate that
you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
themselves.  Default is a 16-bit checksum.  Checksum is calculated by
summing numeric values of expanded values (for string fields the sum of
C<ord($char)> is taken, for bit fields the sum of zeroes and ones).

For example, the following
computes the same number as the System V sum program:

    $checksum = do {
        local $/;  # slurp!
        unpack("%32C*",<>) % 65535;
    };

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);

The C<p> and C<P> formats should be used with care.  Since Perl
has no way of checking whether the value passed to C<unpack()>
corresponds to a valid memory location, passing a pointer value that's
not known to be valid is likely to have disastrous consequences.

If the repeat count of a field is larger than what the remainder of
the input string allows, repeat count is decreased.  If the input string
is longer than one described by the TEMPLATE, the rest is ignored. 

See L</pack> for more examples and notes.

=item untie VARIABLE

Breaks the binding between a variable and a package.  (See C<tie>.)

=item unshift ARRAY,LIST

Does the opposite of a C<shift>.  Or the opposite of a C<push>,
depending on how you look at it.  Prepends list to the front of the
array, and returns the new number of elements in the array.

    unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the
prepended elements stay in the same order.  Use C<reverse> to do the
reverse.

=item use Module LIST

=item use Module

=item use Module VERSION LIST

=item use VERSION

Imports some semantics into the current package from the named module,
generally by aliasing certain subroutine or variable names into your
package.  It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

except that Module I<must> be a bareword.

If the first argument to C<use> is a number, it is treated as a version
number instead of a module name.  If the version of the Perl interpreter
is less than VERSION, then an error message is printed and Perl exits
immediately.  This is often useful if you need to check the current
Perl version before C<use>ing library modules that have changed in
incompatible ways from older versions of Perl.  (We try not to do
this more than we have to.)

The C<BEGIN> forces the C<require> and C<import> to happen at compile time.  The
C<require> makes sure the module is loaded into memory if it hasn't been
yet.  The C<import> is not a builtin--it's just an ordinary static method
call into the C<Module> package to tell the module to import the list of
features back into the current package.  The module can implement its
C<import> method any way it likes, though most modules just choose to
derive their C<import> method via inheritance from the C<Exporter> class that
is defined in the C<Exporter> module.  See L<Exporter>.  If no C<import>
method can be found then the error is currently silently ignored.  This
may change to a fatal error in a future version.

If you don't want your namespace altered, explicitly supply an empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then the
C<use> will call the VERSION method in class Module with the given
version as an argument.  The default VERSION method, inherited from
the Universal class, croaks if the given version is larger than the
value of the variable C<$Module::VERSION>.  (Note that there is not a
comma after VERSION!)

Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way.  Currently implemented pragmas are:

    use integer;
    use diagnostics;
    use sigtrap  qw(SEGV BUS);
    use strict   qw(subs vars refs);
    use subs     qw(afunc blurfl);
    use warnings qw(all);

Some of these pseudo-modules import semantics into the current
block scope (like C<strict> or C<integer>, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).

There's a corresponding C<no> command that unimports meanings imported
by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.

    no integer;
    no strict 'refs';
    no warnings;

If no C<unimport> method can be found the call fails with a fatal error.

See L<perlmod> for a list of standard modules and pragmas.

=item utime LIST

Changes the access and modification times on each file of a list of
files.  The first two elements of the list must be the NUMERICAL access
and modification times, in that order.  Returns the number of files
successfully changed.  The inode change time of each file is set
to the current time.  This code has the same effect as the C<touch>
command if the files already exist:

    #!/usr/bin/perl
    $now = time;
    utime $now, $now, @ARGV;

=item values HASH

Returns a list consisting of all the values of the named hash.  (In a
scalar context, returns the number of values.)  The values are
returned in an apparently random order.  The actual random order is
subject to change in future versions of perl, but it is guaranteed to
be the same order as either the C<keys> or C<each> function would
produce on the same (unmodified) hash.

Note that you cannot modify the values of a hash this way, because the
returned list is just a copy.  You need to use a hash slice for that, 
since it's lvaluable in a way that values() is not.

    for (values %hash)      { s/foo/bar/g }   # FAILS!
    for (@hash{keys %hash}) { s/foo/bar/g }   # ok

As a side effect, calling values() resets the HASH's internal iterator.
See also C<keys>, C<each>, and C<sort>.

=item vec EXPR,OFFSET,BITS

Treats the string in EXPR as a bit vector made up of elements of
width BITS, and returns the value of the element specified by OFFSET
as an unsigned integer.  BITS therefore specifies the number of bits
that are reserved for each element in the bit vector.  This must
be a power of two from 1 to 32 (or 64, if your platform supports
that).

If BITS is 8, "elements" coincide with bytes of the input string.  

If BITS is 16 or more, bytes of the input string are grouped into chunks
of size BITS/8, and each group is converted to a number as with
pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
for BITS==64).  See L<"pack"> for details.

If bits is 4 or less, the string is broken into bytes, then the bits
of each byte are broken into 8/BITS groups.  Bits of a byte are
numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>.  For example,
breaking the single input byte C<chr(0x36)> into two groups gives a list
C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.

C<vec> may also be assigned to, in which case parentheses are needed
to give the expression the correct precedence as in

    vec($image, $max_x * $x + $y, 8) = 3;

Vectors created with C<vec> can also be manipulated with the logical
operators C<|>, C<&>, and C<^>, which will assume a bit vector
operation is desired when both operands are strings.
See L<perlop/"Bitwise String Operators">.

The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
The comments show the string after each step.  Note that this code works
in the same way on big-endian or little-endian machines.

    my $foo = '';
    vec($foo,  0, 32) = 0x5065726C;     # 'Perl'

    # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
    print vec($foo, 0, 8);              # prints 80 == 0x50 == ord('P')

    vec($foo,  2, 16) = 0x5065;         # 'PerlPe'
    vec($foo,  3, 16) = 0x726C;         # 'PerlPerl'
    vec($foo,  8,  8) = 0x50;           # 'PerlPerlP'
    vec($foo,  9,  8) = 0x65;           # 'PerlPerlPe'
    vec($foo, 20,  4) = 2;              # 'PerlPerlPe'   . "\x02"
    vec($foo, 21,  4) = 7;              # 'PerlPerlPer'
                                        # 'r' is "\x72"
    vec($foo, 45,  2) = 3;              # 'PerlPerlPer'  . "\x0c"
    vec($foo, 93,  1) = 1;              # 'PerlPerlPer'  . "\x2c"
    vec($foo, 94,  1) = 1;              # 'PerlPerlPerl'
                                        # 'l' is "\x6c"

To transform a bit vector into a string or list of 0's and 1's, use these:

    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the C<*>.

Here is an example to illustrate how the bits actually fall in place:

    #!/usr/bin/perl -wl

    print <<'EOT';
                                      0         1         2         3  
                       unpack("V",$_) 01234567890123456789012345678901
    ------------------------------------------------------------------
    EOT

    for $w (0..3) {
        $width = 2**$w;
        for ($shift=0; $shift < $width; ++$shift) {
            for ($off=0; $off < 32/$width; ++$off) {
                $str = pack("B*", "0"x32);
                $bits = (1<<$shift);
                vec($str, $off, $width) = $bits;
                $res = unpack("b*",$str);
                $val = unpack("V", $str);
                write;
            }
        }
    }

    format STDOUT =
    vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
    $off, $width, $bits, $val, $res
    .
    __END__

Regardless of the machine architecture on which it is run, the above
example should print the following table:

                                      0         1         2         3  
                       unpack("V",$_) 01234567890123456789012345678901
    ------------------------------------------------------------------
    vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
    vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
    vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
    vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
    vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
    vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
    vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
    vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
    vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
    vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
    vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
    vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
    vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
    vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
    vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
    vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
    vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
    vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
    vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
    vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
    vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
    vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
    vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
    vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
    vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
    vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
    vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
    vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
    vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
    vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
    vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
    vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
    vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
    vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
    vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
    vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
    vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
    vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
    vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
    vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
    vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
    vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
    vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
    vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
    vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
    vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
    vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
    vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
    vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
    vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
    vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
    vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
    vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
    vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
    vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
    vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
    vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
    vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
    vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
    vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
    vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
    vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
    vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
    vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
    vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
    vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
    vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
    vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
    vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
    vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
    vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
    vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
    vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
    vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
    vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
    vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
    vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
    vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
    vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
    vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
    vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
    vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
    vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
    vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
    vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
    vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
    vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
    vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
    vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
    vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
    vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
    vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
    vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
    vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
    vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
    vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
    vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
    vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
    vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
    vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
    vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
    vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
    vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
    vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
    vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
    vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
    vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
    vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
    vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
    vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
    vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
    vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
    vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
    vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
    vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
    vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
    vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
    vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
    vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
    vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
    vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
    vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
    vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
    vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
    vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001

=item wait

Behaves like the wait(2) system call on your system: it waits for a child
process to terminate and returns the pid of the deceased process, or
C<-1> if there are no child processes.  The status is returned in C<$?>.
Note that a return value of C<-1> could mean that child processes are
being automatically reaped, as described in L<perlipc>.

=item waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid of
the deceased process, or C<-1> if there is no such child process.  On some
systems, a value of 0 indicates that there are processes still running.
The status is returned in C<$?>.  If you say

    use POSIX ":sys_wait_h";
    #...
    do { 
        $kid = waitpid(-1,&WNOHANG);
    } until $kid == -1;

then you can do a non-blocking wait for all pending zombie processes.
Non-blocking wait is available on machines supporting either the
waitpid(2) or wait4(2) system calls.  However, waiting for a particular
pid with FLAGS of C<0> is implemented everywhere.  (Perl emulates the
system call by remembering the status values of processes that have
exited but have not been harvested by the Perl script yet.)

Note that on some systems, a return value of C<-1> could mean that child
processes are being automatically reaped.  See L<perlipc> for details,
and for other examples.

=item wantarray

Returns true if the context of the currently executing subroutine is
looking for a list value.  Returns false if the context is looking
for a scalar.  Returns the undefined value if the context is looking
for no value (void context).

    return unless defined wantarray;    # don't bother doing more
    my @a = complex_calculation();
    return wantarray ? @a : "@a";

This function should have been named wantlist() instead.

=item warn LIST

Produces a message on STDERR just like C<die>, but doesn't exit or throw
an exception.

If LIST is empty and C<$@> already contains a value (typically from a
previous eval) that value is used after appending C<"\t...caught">
to C<$@>.  This is useful for staying almost, but not entirely similar to
C<die>.

If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.

No message is printed if there is a C<$SIG{__WARN__}> handler
installed.  It is the handler's responsibility to deal with the message
as it sees fit (like, for instance, converting it into a C<die>).  Most
handlers must therefore make arrangements to actually display the
warnings that they are not prepared to deal with, by calling C<warn>
again in the handler.  Note that this is quite safe and will not
produce an endless loop, since C<__WARN__> hooks are not called from
inside one.

You will find this behavior is slightly different from that of
C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
instead call C<die> again to change it).

Using a C<__WARN__> handler provides a powerful way to silence all
warnings (even the so-called mandatory ones).  An example:

    # wipe out *all* compile-time warnings
    BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
    my $foo = 10;
    my $foo = 20;          # no warning about duplicate my $foo,
                           # but hey, you asked for it!
    # no compile-time or run-time warnings before here
    $DOWARN = 1;

    # run-time warnings enabled after here
    warn "\$foo is alive and $foo!";     # does show up

See L<perlvar> for details on setting C<%SIG> entries, and for more
examples.  See the Carp module for other kinds of warnings using its
carp() and cluck() functions.

=item write FILEHANDLE

=item write EXPR

=item write

Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
using the format associated with that file.  By default the format for
a file is the one having the same name as the filehandle, but the
format for the current output channel (see the C<select> function) may be set
explicitly by assigning the name of the format to the C<$~> variable.

Top of form processing is handled automatically:  if there is
insufficient room on the current page for the formatted record, the
page is advanced by writing a form feed, a special top-of-page format
is used to format the new page header, and then the record is written.
By default the top-of-page format is the name of the filehandle with
"_TOP" appended, but it may be dynamically set to the format of your
choice by assigning the name to the C<$^> variable while the filehandle is
selected.  The number of lines remaining on the current page is in
variable C<$->, which can be set to C<0> to force a new page.

If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
C<select> operator.  If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time.  For more on formats, see L<perlform>.

Note that write is I<not> the opposite of C<read>.  Unfortunately.

=item y///

The transliteration operator.  Same as C<tr///>.  See L<perlop>.

=back