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

=head1 NAME

perldelta - what's new for perl v5.6 (as of v5.005_64)

=head1 DESCRIPTION

This is an unsupported alpha release, meant for intrepid Perl developers
only.  The included sources may not even build correctly on some platforms.
Subscribing to perl5-porters is the best way to monitor and contribute
to the progress of development releases (see www.perl.org for info).

This document describes differences between the 5.005 release and this one.

=head1 Incompatible Changes

=head2 Perl Source Incompatibilities

Beware that any new warnings that have been added or old ones
that have been enhanced are B<not> considered incompatible changes.

Since all new warnings must be explicitly requested via the C<-w>
switch or the C<warnings> pragma, it is ultimately the programmer's
responsibility to ensure that warnings are enabled judiciously.

=over 4

=item STOP is a new keyword

In addition to C<BEGIN>, C<INIT>, C<END>, C<DESTROY> and C<AUTOLOAD>,
subroutines named C<STOP> are now special.  These are queued up during
compilation and behave similar to END blocks, except they are called at
the end of compilation rather than at the end of execution.  They cannot
be called directly.

=item Treatment of list slices of undef has changed

When taking a slice of a literal list (as opposed to a slice of
an array or hash), Perl used to return an empty list if the
result happened to be composed of all undef values.

The new behavior is to produce an empty list if (and only if)
the original list was empty.  Consider the following example:

    @a = (1,undef,undef,2)[2,1,2];

The old behavior would have resulted in @a having no elements.
The new behavior ensures it has three undefined elements.

Note in particular that the behavior of slices of the following
cases remains unchanged:

    @a = ()[1,2];
    @a = (getpwent)[7,0];
    @a = (anything_returning_empty_list())[2,1,2];
    @a = @b[2,1,2];
    @a = @c{'a','b','c'};

See L<perldata>.

=item Possibly changed pseudo-random number generator

In 5.005_0x and earlier, perl's rand() function used the C library
rand(3) function.  As of 5.005_52, Configure tests for drand48(),
random(), and rand() (in that order) and picks the first one it finds.
Perl programs that depend on reproducing a specific set of pseudo-random
numbers will now likely produce different output.  You can use
C<sh Configure -Drandfunc=rand> to obtain the old behavior.

=item Hashing function for hash keys has changed

Perl hashes are not order preserving.  The apparently random order
encountered when iterating on the contents of a hash is determined
by the hashing algorithm used.  To improve the distribution of lower
bits in the hashed value, the algorithm has changed slightly as of
5.005_52.  When iterating over hashes, this may yield a random order
that is B<different> from that of previous versions.

=item C<undef> fails on read only values

Using the C<undef> operator on a readonly value (such as $1) has
the same effect as assigning C<undef> to the readonly value--it
throws an exception.

=item Close-on-exec bit may be set on pipe() handles

On systems that support a close-on-exec flag on filehandles, the
flag will be set for any handles created by pipe(), if that is
warranted by the value of $^F that may be in effect.  Earlier
versions neglected to set the flag for handles created with
pipe().  See L<perlfunc/pipe> and L<perlvar/$^F>.

=item Writing C<"$$1"> to mean C<"${$}1"> is unsupported

Perl 5.004 deprecated the interpretation of C<$$1> and
similar within interpolated strings to mean C<$$ . "1">,
but still allowed it.

In Perl 5.6 and later, C<"$$1"> always means C<"${$1}">.

=item delete(), values() and C<\(%h)> operate on aliases to values, not copies

delete(), each(), values() and hashes in a list context return the actual
values in the hash, instead of copies (as they used to in earlier
versions).  Typical idioms for using these constructs copy the
returned values, but this can make a significant difference when
creating references to the returned values.

Keys in the hash are still returned as copies when iterating on
a hash.

=item vec(EXPR,OFFSET,BITS) enforces powers-of-two BITS

vec() generates a run-time error if the BITS argument is not
a valid power-of-two integer.

=item Text of some diagnostic output has changed

Most references to internal Perl operations in diagnostics
have been changed to be more descriptive.  This may be an
issue for programs that may incorrectly rely on the exact
text of diagnostics for proper functioning.

=item C<%@> has been removed

The undocumented special variable C<%@> that used to accumulate
"background" errors (such as those that happen in DESTROY())
has been removed, because it could potentially result in memory
leaks.

=item Parenthesized not() behaves like a list operator

The C<not> operator now falls under the "if it looks like a function,
it behaves like a function" rule.

As a result, the parenthesized form can be used with C<grep> and C<map>.
The following construct used to be a syntax error before, but it works
as expected now:

    grep not($_), @things;

On the other hand, using C<not> with a literal list slice may not
work.  The following previously allowed construct:

    print not (1,2,3)[0];

needs to be written with additional parentheses now:

    print not((1,2,3)[0]);

The behavior remains unaffected when C<not> is not followed by parentheses.

=back

=head2 C Source Incompatibilities

=over 4

=item C<PERL_POLLUTE>

Release 5.005 grandfathered old global symbol names by providing preprocessor
macros for extension source compatibility.  As of release 5.6, these
preprocessor definitions are not available by default.  You need to explicitly
compile perl with C<-DPERL_POLLUTE> to get these definitions.  For
extensions still using the old symbols, this option can be
specified via MakeMaker:

    perl Makefile.PL POLLUTE=1

=item C<PERL_IMPLICIT_CONTEXT>

PERL_IMPLICIT_CONTEXT is automatically enabled whenever Perl is built
with one of -Dusethreads, -Dusemultiplicity, or both.  It is not
intended to be enabled by users at this time.

This new build option provides a set of macros for all API functions
such that an implicit interpreter/thread context argument is passed to
every API function.  As a result of this, something like C<sv_setsv(foo,bar)>
amounts to a macro invocation that actually translates to something like
C<Perl_sv_setsv(my_perl,foo,bar)>.  While this is generally expected
to not have any significant source compatibility issues, the difference
between a macro and a real function call will need to be considered.

This means that there B<is> a source compatibility issue as a result of
this if your extensions attempt to use pointers to any of the Perl API
functions.

Note that the above issue is not relevant to the default build of
Perl, whose interfaces continue to match those of prior versions
(but subject to the other options described here).

See L<perlguts/"The Perl API"> for detailed information on the
ramifications of building Perl using this option.

=item C<PERL_POLLUTE_MALLOC>

Enabling Perl's malloc in release 5.005 and earlier caused
the namespace of system versions of the malloc family of functions to
be usurped by the Perl versions, since by default they used the
same names.

Besides causing problems on platforms that do not allow these functions to
be cleanly replaced, this also meant that the system versions could not
be called in programs that used Perl's malloc.  Previous versions of Perl
have allowed this behaviour to be suppressed with the HIDEMYMALLOC and
EMBEDMYMALLOC preprocessor definitions.

As of release 5.6, Perl's malloc family of functions have default names
distinct from the system versions.  You need to explicitly compile perl with
C<-DPERL_POLLUTE_MALLOC> to get the older behaviour.  HIDEMYMALLOC
and EMBEDMYMALLOC have no effect, since the behaviour they enabled is now
the default.

Note that these functions do B<not> constitute Perl's memory allocation API.
See L<perlguts/"Memory Allocation"> for further information about that.

=back

=head2 Compatible C Source API Changes

=over

=item C<PATCHLEVEL> is now C<PERL_VERSION>

The cpp macros C<PERL_REVISION>, C<PERL_VERSION>, and C<PERL_SUBVERSION>
are now available by default from perl.h, and reflect the base revision,
patchlevel, and subversion respectively.  C<PERL_REVISION> had no
prior equivalent, while C<PERL_VERSION> and C<PERL_SUBVERSION> were
previously available as C<PATCHLEVEL> and C<SUBVERSION>.

The new names cause less pollution of the B<cpp> namespace and reflect what
the numbers have come to stand for in common practice.  For compatibility,
the old names are still supported when F<patchlevel.h> is explicitly
included (as required before), so there is no source incompatibility
from the change.

=item Support for C++ exceptions

change#3386, also needs perlguts documentation
[TODO - Chip Salzenberg <chip@perlsupport.com>]

=back

=head2 Binary Incompatibilities

The default build of this release is binary compatible with the 5.005
release or its maintenance versions.

The usethreads or usemultiplicity builds are B<not> binary compatible
with the corresponding builds in 5.005.

=head1 Installation and Configuration Improvements

=head2 New Configure flags

The following new flags may be enabled on the Configure command line
by running Configure with C<-Dflag>.

    usemultiplicity

    uselongdouble
    usemorebits
    uselargefiles

=head2 -Dusethreads and -Duse64bits now more daring

The Configure options enabling the use of threads and the use of
64-bitness are now more daring in the sense that they no more have
an explicit list of operating systems of known threads/64-bit
capabilities.  In other words: if your operating system has the
necessary APIs, you should be able just to go ahead and use them.
See also L<"64-bit support">.

=head2 Long Doubles

Some platforms have "long doubles", floating point numbers of even
larger range than ordinary "doubles".  To enable using long doubles for
Perl's scalars, use -Duselongdouble.

=head2 -Dusemorebits

You can enable both -Duse64bits and -Dlongdouble by -Dusemorebits.
See also L<"64-bit support">.

=head2 -Duselargefiles

Some platforms support large files, files larger than two gigabytes.
See L<"Large file support"> for more information. 

=head2 installusrbinperl

You can use "Configure -Uinstallusrbinperl" which causes installperl
to skip installing perl also as /usr/bin/perl.  This is useful if you
prefer not to modify /usr/bin for some reason or another but harmful
because many scripts assume to find Perl in /usr/bin/perl.

=head2 SOCKS support

You can use "Configure -Dusesocks" which causes Perl to probe
for the SOCKS (v5, not v4) proxy protocol library,
http://www.socks.nec.com/

=head2 C<-A> flag

You can "post-edit" the Configure variables using the Configure C<-A>
flag.  The editing happens immediately after the platform specific
hints files have been processed but before the actual configuration
process starts.  Run C<Configure -h> to find out the full C<-A> syntax.

=head2 Enhanced Installation Directories

The installation structure has been enriched to improve the support for 
maintaining multiple versions of perl, to provide locations for
vendor-supplied modules and scripts, and to ease maintenance of
locally-added modules and scripts.  See the section on Installation 
Directories in the INSTALL file for complete details.  For most users
building and installing from source, the defaults should be fine.

=head1 Core Changes

=head2 Unicode and UTF-8 support

Perl can optionally use UTF-8 as its internal representation for character
strings.  The C<utf8> pragma enables this support in the current lexical
scope.  See L<utf8> for more information.

=head2 Interpreter threads

WARNING: This is an experimental feature in a pre-alpha state.  Use
at your own risk.

Perl 5.005_63 introduces the beginnings of support for running multiple
interpreters concurrently in different threads.  In conjunction with
the perl_clone() API call, which can be used to selectively duplicate
the state of any given interpreter, it is possible to compile a
piece of code once in an interpreter, clone that interpreter
one or more times, and run all the resulting interpreters in distinct
threads.

On Windows, this feature is used to emulate fork() at the interpreter
level.  See L<perlfork>.

This feature is still in evolution.  It is eventually meant to be used
to selectively clone a subroutine and data reachable from that
subroutine in a separate interpreter and run the cloned subroutine
in a separate thread.  Since there is no shared data between the
interpreters, little or no locking will be needed (unless parts of
the symbol table are explicitly shared).  This is obviously intended
to be an easy-to-use replacement for the existing threads support.

Support for cloning interpreters must currently be manually enabled
by defining the cpp macro USE_ITHREADS on non-Windows platforms.
(See win32/Makefile for how to enable it on Windows.)  The resulting
perl executable will be functionally identical to one that was built
without USE_ITHREADS, but the perl_clone() API call will only be
available in the former.

USE_ITHREADS enables Perl source code changes that provide a clear
separation between the op tree and the data it operates with.  The
former is considered immutable, and can therefore be shared between
an interpreter and all of its clones, while the latter is considered
local to each interpreter, and is therefore copied for each clone.

Note that building Perl with the -Dusemultiplicity Configure option
is adequate if you wish to run multiple B<independent> interpreters
concurrently in different threads.  USE_ITHREADS only needs to be
enabled if you wish to obtain access to perl_clone() and cloned
interpreters.

[XXX TODO - the Compiler backends may be broken when USE_ITHREADS is
enabled.]

=head2 Lexically scoped warning categories

You can now control the granularity of warnings emitted by perl at a finer
level using the C<use warnings> pragma.  See L<warnings> and L<perllexwarn>
for details.

=head2 Lvalue subroutines

WARNING: This is an experimental feature.

change#4081
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>,
Tuomas Lukka <lukka@fas.harvard.edu>)]

=head2 "our" declarations

An "our" declaration introduces a value that can be best understood
as a lexically scoped symbolic alias to a global variable in the
current package.  This is mostly useful as an alternative to the
C<vars> pragma, but also provides the opportunity to introduce
typing and other attributes for such variables.  See L<perlfunc/our>.

=head2 Weak references

WARNING: This is an experimental feature.

change#3385, also need perlguts documentation

[TODO - Tuomas Lukka <lukka@fas.harvard.edu>]

=head2 File globbing implemented internally

WARNING: This is currently an experimental feature.  Interfaces and
implementation are likely to change.

Perl now uses the File::Glob implementation of the glob() operator
automatically.  This avoids using an external csh process and the
problems associated with it.

=head2 Binary numbers supported

Binary numbers are now supported as literals, in s?printf formats, and
C<oct()>:

    $answer = 0b101010;
    printf "The answer is: %b\n", oct("0b101010");

=head2 Some arrows may be omitted in calls through references

Perl now allows the arrow to be omitted in many constructs
involving subroutine calls through references.  For example,
C<$foo[10]->('foo')> may now be written C<$foo[10]('foo')>.
This is rather similar to how the arrow may be omitted from
C<$foo[10]->{'foo'}>.  Note however, that the arrow is still
required for C<foo(10)->('bar')>.

=head2 exists() and delete() are supported on array elements

The exists() and delete() builtins now work on simple arrays as well.
The behavior is similar to that on hash elements.

exists() can be used to check whether an array element exists without
autovivifying it.  If the array is tied, the EXISTS() method in the
corresponding tied package will be invoked.

delete() may now be used to remove an element from the array and return
it.  If the element happens to be the one at the end, the size of the
array also shrinks by one.  If the array is tied, the DELETE() method
in the corresponding tied package will be invoked.

See L<perlfunc/exists> and L<perlfunc/delete> for examples.

=head2 syswrite() ease-of-use

The length argument of C<syswrite()> has become optional.

=head2 Filehandles can be autovivified

Similar to how constructs such as C<$x->[0]> autovivify a reference,
open() now autovivifies a filehandle if the first argument is an
uninitialized variable.  This allows the constructs C<open(my $fh, ...)> and
C<open(local $fh,...)> to be used to create filehandles that will
conveniently be closed automatically when the scope ends, provided there
are no other references to them.  This largely eliminates the need for
typeglobs when opening filehandles that must be passed around, as in the
following example:

    sub myopen {
        open my $fh, "@_"
             or die "Can't open '@_': $!";
        return $fh;
    }

    {
        my $f = myopen("</etc/motd");
        print <$f>;
        # $f implicitly closed here
    }

[TODO - this idiom needs more pod penetration]

=head2 64-bit support

All platforms that have 64-bit integers either (a) natively as longs
or ints (b) via special compiler flags (c) using long long are able to
use "quads" (64-integers) as follows:

=over 4

=item *

constants (decimal, hexadecimal, octal, binary) in the code 

=item *

arguments to oct() and hex()

=item *

arguments to print(), printf() and sprintf() (flag prefixes ll, L, q)

=item *

printed as such

=item *

pack() and unpack() "q" and "Q" formats

=item *

in basic arithmetics: + - * / %

=item *

vec() (but see the below note about bit arithmetics)

=back

Note that unless you have the case (a) you will have to configure
and compile Perl using the -Duse64bits Configure flag.

Unfortunately bit arithmetics (&, |, ^, ~, <<, >>) for numbers are not
64-bit clean, they are explictly forced to be 32-bit.  Bit arithmetics
for bit vectors (created by vec()) are not limited in their width.

Last but not least: note that due to Perl's habit of always using
floating point numbers the quads are still not true integers.
When quads overflow their limits (0...18_446_744_073_709_551_615 unsigned,
-9_223_372_036_854_775_808...9_223_372_036_854_775_807 signed), they
are silently promoted to floating point numbers, after which they will
start losing precision (their lower digits).

=head2 Large file support

If you have filesystems that support "large files" (files larger than
2 gigabytes), you may now also be able to create and access them from
Perl.  You have to use Configure -Duselargefiles.  Turning on the
large file support turns on also the 64-bit support, for obvious reasons.

Note that in addition to requiring a proper file system to do large
files you may also need to adjust your per-process (or your
per-system, or per-process-group, or per-user-group) maximum filesize
limits before running Perl scripts that try to handle large files,
especially if you intend to write such files.

Finally, in addition to your process/process group maximum filesize
limits, you may have quota limits on your filesystems that stop you
(your user id or your user group id) from using large files.

Adjusting your process/user/group/file system/operating system limits
is outside the scope of Perl core language.  For process limits, you
may try increasing the limits using your shell's limits/limit/ulimit
command before running Perl.  The BSD::Resource extension (not
included with the standard Perl distribution) may also be of use, it
offers the getrlimit/setrlimit interface that can be used to adjust
process resource usage limits, including the maximum filesize limit.

=head2 Long doubles

In some systems you may be able to use long doubles to enhance the
range of precision of your double precision floating point numbers
(that is, Perl's numbers).  Use Configure -Duselongdouble to enable
this support (if it is available).

=head2 "more bits"

You can Configure -Dusemorebits to turn on both the 64-bit support
and the long double support.

=head2 Enhanced support for sort() subroutines

Perl subroutines with a prototype of C<($$)> and XSUBs in general can
now be used as sort subroutines.  In either case, the two elements to
be compared are passed as normal parameters in @_.  See L<perlfunc/sort>.

For unprototyped sort subroutines, the historical behavior of passing 
the elements to be compared as the global variables $a and $b remains
unchanged.

=head2 Better syntax checks on parenthesized unary operators

Expressions such as:

    print defined(&foo,&bar,&baz);
    print uc("foo","bar","baz");
    undef($foo,&bar);

used to be accidentally allowed in earlier versions, and produced
unpredictable behaviour.  Some produced ancillary warnings
when used in this way; others silently did the wrong thing.

The parenthesized forms of most unary operators that expect a single
argument now ensure that they are not called with more than one
argument, making the cases shown above syntax errors.  The usual
behaviour of:

    print defined &foo, &bar, &baz;
    print uc "foo", "bar", "baz";
    undef $foo, &bar;

remains unchanged.  See L<perlop>.

=head2 POSIX character class syntax [: :] supported

For example to match alphabetic characters use /[[:alpha:]]/.
See L<perlre> for details.

=head2 Improved C<qw//> operator

The C<qw//> operator is now evaluated at compile time into a true list
instead of being replaced with a run time call to C<split()>.  This
removes the confusing misbehaviour of C<qw//> in scalar context, which
had inherited that behaviour from split().

Thus:

    $foo = ($bar) = qw(a b c); print "$foo|$bar\n";

now correctly prints "3|a", instead of "2|a".

=head2 pack() format 'Z' supported

The new format type 'Z' is useful for packing and unpacking null-terminated
strings.  See L<perlfunc/"pack">.

=head2 pack() format modifier '!' supported

The new format type modifier '!' is useful for packing and unpacking
native shorts, ints, and longs.  See L<perlfunc/"pack">.

=head2 pack() and unpack() support counted strings

The template character '/' can be used to specify a counted string
type to be packed or unpacked.  See L<perlfunc/"pack">.

=head2 Comments in pack() templates

The '#' character in a template introduces a comment up to
end of the line.  This facilitates documentation of pack()
templates.

=head2 $^X variables may now have names longer than one character

Formerly, $^X was synonymous with ${"\cX"}, but $^XY was a syntax
error.  Now variable names that begin with a control character may be
arbitrarily long.  However, for compatibility reasons, these variables
I<must> be written with explicit braces, as C<${^XY}> for example.
C<${^XYZ}> is synonymous with ${"\cXYZ"}.  Variable names with more
than one control character, such as C<${^XY^Z}>, are illegal.

The old syntax has not changed.  As before, `^X' may be either a
literal control-X character or the two-character sequence `caret' plus
`X'.  When braces are omitted, the variable name stops after the
control character.  Thus C<"$^XYZ"> continues to be synonymous with
C<$^X . "YZ"> as before.

As before, lexical variables may not have names beginning with control
characters.  As before, variables whose names begin with a control
character are always forced to be in package `main'.  All such variables
are reserved for future extensions, except those that begin with
C<^_>, which may be used by user programs and are guaranteed not to
acquire special meaning in any future version of Perl.

=head2 C<use attrs> implicit in subroutine attributes

Formerly, if you wanted to mark a subroutine as being a method call or
as requiring an automatic lock() when it is entered, you had to declare
that with a C<use attrs> pragma in the body of the subroutine.
That can now be accomplished with a declaration syntax, like this:

    sub mymethod : locked, method ;
    ...
    sub mymethod : locked, method {
        ...
    }

F<AutoSplit.pm> and F<SelfLoader.pm> have been updated to keep the attributes
with the stubs they provide.  See L<attributes>.

=head2 Regular expression improvements

change#2827,2373,2372,2365,1813,1800,4112,4158,4215,4301
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 Overloading improvements

change#2150
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 open() with more than two arguments

[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 Support for interpolating named characters

change#4052
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 Experimental support for user-hooks in @INC

[TODO - Ken Fox <kfox@ford.com>]

=head2 C<require> and C<do> may be overridden

C<require> and C<do 'file'> operations may be overridden locally
by importing subroutines of the same name into the current package 
(or globally by importing them into the CORE::GLOBAL:: namespace).
Overriding C<require> will also affect C<use>, provided the override
is visible at compile-time.
See L<perlsub/"Overriding Built-in Functions">.

=head2 New variable $^C reflects C<-c> switch

C<$^C> has a boolean value that reflects whether perl is being run
in compile-only mode (i.e. via the C<-c> switch).  Since
BEGIN blocks are executed under such conditions, this variable
enables perl code to determine whether actions that make sense
only during normal running are warranted.  See L<perlvar>.

=head2 Optional Y2K warnings

If Perl is built with the cpp macro C<PERL_Y2KWARN> defined,
it emits optional warnings when concatenating the number 19
with another number.

This behavior must be specifically enabled when running Configure.
See L<INSTALL> and L<README.Y2K>.

=head1 Significant bug fixes

=head2 E<lt>HANDLEE<gt> on empty files

With C<$/> set to C<undef>, slurping an empty file returns a string of
zero length (instead of C<undef>, as it used to) the first time the
HANDLE is read.  Further reads yield C<undef>.

This means that the following will append "foo" to an empty file (it used
to do nothing):

    perl -0777 -pi -e 's/^/foo/' empty_file

The behaviour of:

    perl -pi -e 's/^/foo/' empty_file

is unchanged (it continues to leave the file empty).

=head2 C<eval '...'> improvements

Line numbers (as reflected by caller() and most diagnostics) within
C<eval '...'> were often incorrect when here documents were involved.
This has been corrected.

Lexical lookups for variables appearing in C<eval '...'> within
functions that were themselves called within an C<eval '...'> were
searching the wrong place for lexicals.  The lexical search now
correctly ends at the subroutine's block boundary.

Parsing of here documents used to be flawed when they appeared as
the replacement expression in C<eval 's/.../.../e'>.  This has
been fixed.

=head2 All compilation errors are true errors

Some "errors" encountered at compile time were by neccessity 
generated as warnings followed by eventual termination of the
program.  This enabled more such errors to be reported in a
single run, rather than causing a hard stop at the first error
that was encountered.

The mechanism for reporting such errors has been reimplemented
to queue compile-time errors and report them at the end of the
compilation as true errors rather than as warnings.  This fixes
cases where error messages leaked through in the form of warnings
when code was compiled at run time using C<eval STRING>, and
also allows such errors to be reliably trapped using __DIE__ hooks.

=head2 Automatic flushing of output buffers

fork(), exec(), system(), qx//, and pipe open()s now flush buffers
of all files opened for output when the operation
was attempted.  This mostly eliminates confusing 
buffering mishaps suffered by users unaware of how Perl internally
handles I/O.

=head2 Better diagnostics on meaningless filehandle operations

Constructs such as C<open(E<lt>FHE<gt>)> and C<close(E<lt>FHE<gt>)>
are compile time errors.  Attempting to read from filehandles that
were opened only for writing will now produce warnings (just as
writing to read-only filehandles does).

=head2 Where possible, buffered data discarded from duped input filehandle

C<open(NEW, "E<lt>&OLD")> now attempts to discard any data that
was previously read and buffered in C<OLD> before duping the handle.
On platforms where doing this is allowed, the next read operation
on C<NEW> will return the same data as the corresponding operation
on C<OLD>.  Formerly, it would have returned the data from the start
of the following disk block instead.

=head2 eof() has the same old magic as <>

C<eof()> would return true if no attempt to read from C<E<lt>E<gt>> had
yet been made.  C<eof()> has been changed to have a little magic of its
own, it now opens the C<E<lt>E<gt>> files.

=head2 system(), backticks and pipe open now reflect exec() failure

On Unix and similar platforms, system(), qx() and open(FOO, "cmd |")
etc., are implemented via fork() and exec().  When the underlying
exec() fails, earlier versions did not report the error properly,
since the exec() happened to be in a different process.

The child process now communicates with the parent about the
error in launching the external command, which allows these
constructs to return with their usual error value and set $!.

=head2 Implicitly closed filehandles are safer

Sometimes implicitly closed filehandles (as when they are localized,
and Perl automatically closes them on exiting the scope) could
inadvertently set $? or $!.  This has been corrected.

=head2 C<(\$)> prototype and C<$foo{a}>

An scalar reference prototype now correctly allows a hash or
array element in that slot.

=head2 Pseudo-hashes work better

Dereferencing some types of reference values in a pseudo-hash,
such as C<$ph->{foo}[1]>, was accidentally disallowed.  This has
been corrected.

When applied to a pseudo-hash element, exists() now reports whether
the specified value exists, not merely if the key is valid.

delete() now works on pseudo-hashes.  When given a pseudo-hash element
or slice it deletes the values corresponding to the keys (but not the keys
themselves).  See L<perlref/"Pseudo-hashes: Using an array as a hash">.

=head2 C<goto &sub> and AUTOLOAD

The C<goto &sub> construct works correctly when C<&sub> happens
to be autoloaded.

=head2 C<-bareword> allowed under C<use integer>

The autoquoting of barewords preceded by C<-> did not work
in prior versions when the C<integer> pragma was enabled.
This has been fixed.

=head2 Boolean assignment operators are legal lvalues

Constructs such as C<($a ||= 2) += 1> are now allowed.

=head2 C<sort $coderef @foo> allowed

sort() did not accept a subroutine reference as the comparison
function in earlier versions.  This is now permitted.

=head2 Failures in DESTROY()

When code in a destructor threw an exception, it went unnoticed
in earlier versions of Perl, unless someone happened to be
looking in $@ just after the point the destructor happened to
run.  Such failures are now visible as warnings when warnings are
enabled.

=head2 Locale bugs fixed

printf() and sprintf() previously reset the numeric locale
back to the default "C" locale.  This has been fixed.

Numbers formatted according to the local numeric locale
(such as using a decimal comma instead of a decimal dot) caused
"isn't numeric" warnings, even while the operations accessing
those numbers produced correct results.  The warnings are gone.

=head2 Memory leaks

The C<eval 'return sub {...}'> construct could sometimes leak
memory.  This has been fixed.

Operations that aren't filehandle constructors used to leak memory
when used on invalid filehandles.  This has been fixed.

Constructs that modified C<@_> could fail to deallocate values
in C<@_> and thus leak memory.  This has been corrected.

=head2 Spurious subroutine stubs after failed subroutine calls

Perl could sometimes create empty subroutine stubs when a
subroutine was not found in the package.  Such cases stopped
later method lookups from progressing into base packages.
This has been corrected.

=head2 Consistent numeric conversions

change#3378,3318
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 Taint failures under C<-U>

When running in unsafe mode, taint violations could sometimes
cause silent failures.  This has been fixed.

=head2 END blocks and the C<-c> switch

Prior versions used to run BEGIN B<and> END blocks when Perl was
run in compile-only mode.  Since this is typically not the expected
behavior, END blocks are not executed anymore when the C<-c> switch
is used.

See L<STOP blocks> for how to run things when the compile phase ends.

=head2 Potential to leak DATA filehandles

Using the C<__DATA__> token creates an implicit filehandle to
the file that contains the token.  It is the program's
responsibility to close it when it is done reading from it.

This caveat is now better explained in the documentation.
See L<perldata>.

=head2 Diagnostics follow STDERR

Diagnostic output now goes to whichever file the C<STDERR> handle
is pointing at, instead of always going to the underlying C runtime
library's C<stderr>.

=head2 Other fixes for better diagnostics

Line numbers are no longer suppressed (under most likely circumstances)
during the global destruction phase.

Diagnostics emitted from code running in threads other than the main
thread are now accompanied by the thread ID.

Embedded null characters in diagnostics now actually show up.  They
used to truncate the message in prior versions.

$foo::a and $foo::b are now exempt from "possible typo" warnings only
if sort() is encountered in package foo.

Unrecognized alphabetic escapes encountered when parsing quote
constructs now generate a warning, since they may take on new
semantics in later versions of Perl.

=head1 Performance enhancements

=head2 Simple sort() using { $a <=> $b } and the like are optimized

Many common sort() operations using a simple inlined block are now
optimized for faster performance.

=head2 Optimized assignments to lexical variables

Certain operations in the RHS of assignment statements have been
optimized to directly set the lexical variable on the LHS,
eliminating redundant copying overheads.

=head2 Method lookups optimized

[TODO - Chip Salzenberg <chip@perlsupport.com>]

=head2 Faster mechanism to invoke XSUBs

change#4044,4125
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 Perl_malloc() improvements

change#4237
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 Faster subroutine calls

Minor changes in how subroutine calls are handled internally
provide marginal improvements in performance.

=head1 Platform specific changes

=head2 Additional supported platforms

=over 4

=item *

VM/ESA is now supported.

=item *

Siemens BS2000 is now supported under the POSIX Shell.

=item *

The Mach CThreads (NEXTSTEP, OPENSTEP) are now supported by the Thread
extension.

=item *

GNU/Hurd is now supported.

=item *

Rhapsody is now supported.

=item *

EPOC is is now supported (on Psion 5).

=back

=head2 DOS

=over 4

=item *

Perl now works with djgpp 2.02 (and 2.03 alpha).

=item *

Environment variable names are not converted to uppercase any more.

=item *

Wrong exit code from backticks now fixed.

=item *

This port is still using its own builtin globbing.

=back

=head2 OS/2

[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head2 VMS

[TODO - Charles Bailey <bailey@newman.upenn.edu>]

=head2 Win32

Site library searches failed to look for ".../site/5.XXX/lib"
if ".../site/5.XXXYY/lib" wasn't found.  This has been corrected.

When given a pathname that consists only of a drivename, such
as C<A:>, opendir() and stat() now use the current working
directory for the drive rather than the drive root.

The builtin XSUB functions in the Win32:: namespace are
documented.  See L<Win32>.

$^X now contains the full path name of the running executable.

A Win32::GetLongPathName() function is provided to complement
Win32::GetFullPathName() and Win32::GetShortPathName().  See L<Win32>.

POSIX::uname() is supported.

system(1,...) now returns true process IDs rather than process
handles.  kill() accepts any real process id, rather than strictly
return values from system(1,...).

The C<Shell> module is supported.

Rudimentary support for building under command.com in Windows 95
has been added.

Scripts are read in binary mode by default to allow ByteLoader (and
the filter mechanism in general) to work properly.  For compatibility,
the DATA filehandle will be set to text mode if a carriage return is
detected at the end of the line containing the __END__ or __DATA__
token; if not, the DATA filehandle will be left open in binary mode.
Earlier versions always opened the DATA filehandle in text mode.

[TODO - GSAR]

=head1 New tests

=over 4

=item   lib/attrs

Compatibility tests for C<sub : attrs> vs the older C<use attrs>.

=item   lib/io_const

IO constants (SEEK_*, _IO*).

=item   lib/io_dir

Directory-related IO methods (new, read, close, rewind, tied delete).

=item   lib/io_multihomed

INET sockets with multi-homed hosts.

=item   lib/io_poll

IO poll().

=item   lib/io_unix

UNIX sockets.

=item   op/attrs

Regression tests for C<my ($x,@y,%z) : attrs> and <sub : attrs>.

=item   op/filetest

File test operators.

=item   op/lex_assign

Verify operations that access pad objects (lexicals and temporaries).

=back

=head1 Modules and Pragmata

=head2 Modules

=over 4

=item attributes

While used internally by Perl as a pragma, this module also
provides a way to fetch subroutine and variable attributes.
See L<attributes>.

=item B

The Perl Compiler suite has been extensively reworked for this
release.

[TODO - Vishal Bhatia <vishal@gol.com>,
Nick Ing-Simmons <nick@ni-s.u-net.com>]

=item ByteLoader

The ByteLoader is a dedicated extension to generate and run
Perl bytecode.  See L<ByteLoader>.

=item constant

References can now be used.

The new version also allows a leading underscore in constant names, but
disallows a double leading underscore (as in "__LINE__").  Some other names
are disallowed or warned against, including BEGIN, END, etc.  Some names
which were forced into main:: used to fail silently in some cases; now they're
fatal (outside of main::) and an optional warning (inside of main::).
The ability to detect whether a constant had been set with a given name has
been added.

See L<constant>.

=item charnames

change#4052
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=item Data::Dumper

A C<Maxdepth> setting can be specified to avoid venturing
too deeply into deep data structures.  See L<Data::Dumper>.

Dumping C<qr//> objects works correctly.

=item DB

C<DB> is an experimental module that exposes a clean abstraction
to Perl's debugging API.

=item DB_File

DB_File can now be built with Berkeley DB versions 1, 2 or 3.
See C<ext/DB_File/Changes>.

=item Devel::DProf

Devel::DProf, a Perl source code profiler has been added.  See
L<Devel::DProf> and L<dprofpp>.

=item Dumpvalue

The Dumpvalue module provides screen dumps of Perl data.

=item Benchmark

Overall, Benchmark results exhibit lower average error and better timing
accuracy.  

You can now run tests for I<n> seconds instead of guessing the right
number of tests to run: e.g. timethese(-5, ...) will run each 
code for at least 5 CPU seconds.  Zero as the "number of repetitions"
means "for at least 3 CPU seconds".  The output format has also
changed.  For example:

   use Benchmark;$x=3;timethese(-5,{a=>sub{$x*$x},b=>sub{$x**2}})

will now output something like this:

   Benchmark: running a, b, each for at least 5 CPU seconds...
            a:  5 wallclock secs ( 5.77 usr +  0.00 sys =  5.77 CPU) @ 200551.91/s (n=1156516)
            b:  4 wallclock secs ( 5.00 usr +  0.02 sys =  5.02 CPU) @ 159605.18/s (n=800686)

New features: "each for at least N CPU seconds...", "wallclock secs",
and the "@ operations/CPU second (n=operations)".

timethese() now returns a reference to a hash of Benchmark objects containing
the test results, keyed on the names of the tests.

timethis() now returns the iterations field in the Benchmark result object
instead of 0.

timethese(), timethis(), and the new cmpthese() (see below) can also take
a format specifier of 'none' to suppress output.

A new function countit() is just like timeit() except that it takes a
TIME instead of a COUNT.

A new function cmpthese() prints a chart comparing the results of each test
returned from a timethese() call.  For each possible pair of tests, the
percentage speed difference (iters/sec or seconds/iter) is shown.

For other details, see L<Benchmark>.

=item Devel::Peek

The Devel::Peek module provides access to the internal representation
of Perl variables and data.  It is a data debugging tool for the XS programmer.

=item ExtUtils::MakeMaker

change#4135, also needs docs in module pod
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=item Fcntl

More Fcntl constants added: F_SETLK64, F_SETLKW64, O_LARGEFILE for
large (more than 4G) file access (64-bit support is not yet
working, though, so no need to get overly excited), Free/Net/OpenBSD
locking behaviour flags F_FLOCK, F_POSIX, Linux F_SHLCK, and
O_ACCMODE: the mask of O_RDONLY, O_WRONLY, and O_RDWR.

=item File::Compare

A compare_text() function has been added, which allows custom
comparison functions.  See L<File::Compare>.

=item File::Find

File::Find now works correctly when the wanted() function is either
autoloaded or is a symbolic reference.

A bug that caused File::Find to lose track of the working directory
when pruning top-level directories has been fixed.

File::Find now also supports several other options to control its
behavior.  It can follow symbolic links if the C<follow> option is
specified.  Enabling the C<no_chdir> option will make File::Find skip
changing the current directory when walking directories.  The C<untaint>
flag can be useful when running with taint checks enabled.

See L<File::Find>.

=item File::Glob

This extension implements BSD-style file globbing.  By default,
it will also be used for the internal implementation of the glob()
operator.  See L<File::Glob>.

=item File::Spec

New methods have been added to the File::Spec module: devnull() returns
the name of the null device (/dev/null on Unix) and tmpdir() the name of
the temp directory (normally /tmp on Unix).  There are now also methods
to convert between absolute and relative filenames: abs2rel() and
rel2abs().  For compatibility with operating systems that specify volume
names in file paths, the splitpath(), splitdir(), and catdir() methods
have been added.

=item File::Spec::Functions

The new File::Spec::Functions modules provides a function interface
to the File::Spec module.  Allows shorthand

    $fullname = catfile($dir1, $dir2, $file);

instead of

    $fullname = File::Spec->catfile($dir1, $dir2, $file);

=item Getopt::Long

Getopt::Long licensing has changed to allow the Perl Artistic License
as well as the GPL. It used to be GPL only, which got in the way of
non-GPL applications that wanted to use Getopt::Long.

Getopt::Long encourages the use of Pod::Usage to produce help
messages. For example:

    use Getopt::Long;
    use Pod::Usage;
    my $man = 0;
    my $help = 0;
    GetOptions('help|?' => \$help, man => \$man) or pod2usage(2);
    pod2usage(1) if $help;
    pod2usage(-exitstatus => 0, -verbose => 2) if $man;

    __END__

    =head1 NAME

    sample - Using GetOpt::Long and Pod::Usage

    =head1 SYNOPSIS

    sample [options] [file ...]

     Options:
       -help            brief help message
       -man             full documentation

    =head1 OPTIONS

    =over 8

    =item B<-help>

    Print a brief help message and exits.

    =item B<-man>

    Prints the manual page and exits.

    =back

    =head1 DESCRIPTION

    B<This program> will read the given input file(s) and do someting
    useful with the contents thereof.

    =cut

See L<Pod::Usage> for details.

A bug that prevented the non-option call-back E<lt>E<gt> from being
specified as the first argument has been fixed.

To specify the characters E<lt> and E<gt> as option starters, use
E<gt>E<lt>. Note, however, that changing option starters is strongly
deprecated. 

=item IO

write() and syswrite() will now accept a single-argument
form of the call, for consistency with Perl's syswrite().

You can now create a TCP-based IO::Socket::INET without forcing
a connect attempt.  This allows you to configure its options
(like making it non-blocking) and then call connect() manually.

A bug that prevented the IO::Socket::protocol() accessor
from ever returning the correct value has been corrected.

=item JPL

Java Perl Lingo is now distributed with Perl.  See jpl/README
for more information.

=item lib

C<use lib> now weeds out any trailing duplicate entries.
C<no lib> removes all named entries.

=item Math::BigInt

The bitwise operations C<E<lt>E<lt>>, C<E<gt>E<gt>>, C<&>, C<|>,
and C<~> are now supported on bigints.

=item Math::Complex

The accessor methods Re, Im, arg, abs, rho, and theta can now also
act as mutators (accessor $z->Re(), mutator $z->Re(3)).

=item Math::Trig

A little bit of radial trigonometry (cylindrical and spherical),
radial coordinate conversions, and the great circle distance were added.

=item Pod::Parser

[TODO - Brad Appleton <bradapp@enteract.com>]

=item Pod::Text and Pod::Man

[TODO - Russ Allbery <rra@stanford.edu>]

=item SDBM_File

An EXISTS method has been added to this module (and sdbm_exists() has
been added to the underlying sdbm library), so one can now call exists
on an SDBM_File tied hash and get the correct result, rather than a
runtime error.

A bug that may have caused data loss when more than one disk block
happens to be read from the database in a single FETCH() has been
fixed.

=item Time::Local

The timelocal() and timegm() functions used to silently return bogus
results when the date fell outside the machine's integer range.  They
now consistently croak() if the date falls in an unsupported range.

=item Win32

The error return value in list context has been changed for all functions
that return a list of values.  Previously these functions returned a list
with a single element C<undef> if an error occurred.  Now these functions
return the empty list in these situations.  This applies to the following
functions:

    Win32::FsType
    Win32::GetOSVersion

The remaining functions are unchanged and continue to return C<undef> on
error even in list context.

The Win32::SetLastError(ERROR) function has been added as a complement
to the Win32::GetLastError() function.

The new Win32::GetFullPathName(FILENAME) returns the full absolute
pathname for FILENAME in scalar context.  In list context it returns
a two-element list containing the fully qualified directory name and
the filename.  See L<Win32>.

=item DBM Filters

A new feature called "DBM Filters" has been added to all the
DBM modules--DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File.
DBM Filters add four new methods to each DBM module:

    filter_store_key
    filter_store_value
    filter_fetch_key
    filter_fetch_value

These can be used to filter key-value pairs before the pairs are
written to the database or just after they are read from the database.
See L<perldbmfilter> for further information.

=back

=head2 Pragmata

C<use attrs> is now obsolete, and is only provided for
backward-compatibility.  It's been replaced by the C<sub : attributes>
syntax.  See L<perlsub/"Subroutine Attributes"> and L<attributes>.

C<use utf8> to enable UTF-8 and Unicode support.

C<use caller 'encoding'> allows modules to inherit pragmatic attributes
from the caller's context.  C<encoding> is currently the only supported
attribute.

Lexical warnings pragma, C<use warnings;>, to control optional warnings.
See L<perllexwarn>.

C<use filetest> to control the behaviour of filetests (C<-r> C<-w>
...).  Currently only one subpragma implemented, "use filetest
'access';", that uses access(2) or equivalent to check permissions
instead of using stat(2) as usual.  This matters in filesystems
where there are ACLs (access control lists): the stat(2) might lie,
but access(2) knows better.

=head1 Utility Changes

=head2 h2ph

[TODO - Kurt Starsinic <kstar@chapin.edu>]

=head2 perlcc

C<perlcc> now supports the C and Bytecode backends.  By default,
it generates output from the simple C backend rather than the
optimized C backend.

Support for non-Unix platforms has been improved.

=head2 h2xs

change#4232
[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

=head1 Documentation Changes

=over 4

=item perlcompile.pod

An introduction to using the Perl Compiler suite.

=item perlfilter.pod

An introduction to writing Perl source filters.

=item perlhack.pod

Some guidelines for hacking the Perl source code.

=item perlopentut.pod

A tutorial on using open() effectively.

=item perlreftut.pod

A tutorial that introduces the essentials of references.

=item perltootc.pod

A tutorial on managing class data for object modules.

=back

=head1 New or Changed Diagnostics

=over 4

=item "my sub" not yet implemented

(F) Lexically scoped subroutines are not yet implemented.  Don't try that
yet.

=item '!' allowed only after types %s

(F) The '!' is allowed in pack() and unpack() only after certain types.
See L<perlfunc/pack>.

=item / cannot take a count

(F) You had an unpack template indicating a counted-length string,
but you have also specified an explicit size for the string.
See L<perlfunc/pack>.

=item / must be followed by a, A or Z

(F) You had an unpack template indicating a counted-length string,
which must be followed by one of the letters a, A or Z
to indicate what sort of string is to be unpacked.
See L<perlfunc/pack>.

=item / must be followed by a*, A* or Z*

(F) You had a pack template indicating a counted-length string,
Currently the only things that can have their length counted are a*, A* or Z*.
See L<perlfunc/pack>.

=item / must follow a numeric type

(F) You had an unpack template that contained a '#',
but this did not follow some numeric unpack specification.
See L<perlfunc/pack>.

=item /%s/: Unrecognized escape \\%c passed through

(W) You used a backslash-character combination which is not recognized
by Perl.  This combination appears in an interpolated variable or a
C<'>-delimited regular expression.  The character was understood literally.

=item /%s/: Unrecognized escape \\%c in character class passed through

(W) You used a backslash-character combination which is not recognized
by Perl inside character classes.  The character was understood literally.

=item /%s/ should probably be written as "%s"

(W) You have used a pattern where Perl expected to find a string,
as in the first argument to C<join>.  Perl will treat the true
or false result of matching the pattern against $_ as the string,
which is probably not what you had in mind.

=item %s() called too early to check prototype

(W) You've called a function that has a prototype before the parser saw a
definition or declaration for it, and Perl could not check that the call
conforms to the prototype.  You need to either add an early prototype
declaration for the subroutine in question, or move the subroutine
definition ahead of the call to get proper prototype checking.  Alternatively,
if you are certain that you're calling the function correctly, you may put
an ampersand before the name to avoid the warning.  See L<perlsub>.

=item %s package attribute may clash with future reserved word: %s

(W) A lowercase attribute name was used that had a package-specific handler.
That name might have a meaning to Perl itself some day, even though it
doesn't yet.  Perhaps you should use a mixed-case attribute name, instead.
See L<attributes>.

=item         (in cleanup) %s

(W) This prefix usually indicates that a DESTROY() method raised
the indicated exception.  Since destructors are usually called by
the system at arbitrary points during execution, and often a vast
number of times, the warning is issued only once for any number
of failures that would otherwise result in the same message being
repeated.

Failure of user callbacks dispatched using the C<G_KEEPERR> flag
could also result in this warning.  See L<perlcall/G_KEEPERR>.

=item <> should be quotes

(F) You wrote C<require E<lt>fileE<gt>> when you should have written
C<require 'file'>.

=item Attempt to join self

(F) You tried to join a thread from within itself, which is an
impossible task.  You may be joining the wrong thread, or you may
need to move the join() to some other thread.

=item Bad evalled substitution pattern

(F) You've used the /e switch to evaluate the replacement for a
substitution, but perl found a syntax error in the code to evaluate,
most likely an unexpected right brace '}'.

=item Bad realloc() ignored

(S) An internal routine called realloc() on something that had never been
malloc()ed in the first place. Mandatory, but can be disabled by
setting environment variable C<PERL_BADFREE> to 1.

=item Binary number > 0b11111111111111111111111111111111 non-portable

(W) The binary number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
L<perlport> for more on portability concerns.

=item Bit vector size > 32 non-portable

(W) Using bit vector sizes larger than 32 is non-portable.

=item Buffer overflow in prime_env_iter: %s

(W) A warning peculiar to VMS.  While Perl was preparing to iterate over
%ENV, it encountered a logical name or symbol definition which was too long,
so it was truncated to the string shown.

=item Can't check filesystem of script "%s"

(P) For some reason you can't check the filesystem of the script for nosuid.

=item Can't ignore signal CHLD, forcing to default

(W) Perl has detected that it is being run with the SIGCHLD signal
(sometimes known as SIGCLD) disabled.  Since disabling this signal
will interfere with proper determination of exit status of child
processes, Perl has reset the signal to its default value.
This situation typically indicates that the parent program under
which Perl may be running (e.g. cron) is being very careless.

=item Can't modify non-lvalue subroutine call

(F) Subroutines meant to be used in lvalue context should be declared as
such, see L<perlsub/"Lvalue subroutines">.

=item Can't read CRTL environ

(S) A warning peculiar to VMS.  Perl tried to read an element of %ENV
from the CRTL's internal environment array and discovered the array was
missing.  You need to figure out where your CRTL misplaced its environ
or define F<PERL_ENV_TABLES> (see L<perlvms>) so that environ is not searched.

=item Can't remove %s: %s, skipping file 

(S) You requested an inplace edit without creating a backup file.  Perl
was unable to remove the original file to replace it with the modified
file.  The file was left unmodified.

=item Can't return %s from lvalue subroutine

(F) Perl detected an attempt to return illegal lvalues (such
as temporary or readonly values) from a subroutine used as an lvalue.
This is not allowed.

=item Can't weaken a nonreference

(F) You attempted to weaken something that was not a reference.  Only
references can be weakened.

=item Character class [:%s:] unknown

(F) The class in the character class [: :] syntax is unknown.
See L<perlre>.

=item Character class syntax [%s] belongs inside character classes

(W) The character class constructs [: :], [= =], and [. .]  go
I<inside> character classes, the [] are part of the construct,
for example: /[012[:alpha:]345]/.  Note that [= =] and [. .]
are not currently implemented; they are simply placeholders for
future extensions.

=item Constant is not %s reference

(F) A constant value (perhaps declared using the C<use constant> pragma)
is being dereferenced, but it amounts to the wrong type of reference.  The
message indicates the type of reference that was expected. This usually
indicates a syntax error in dereferencing the constant value.
See L<perlsub/"Constant Functions"> and L<constant>.

=item constant(%s): %%^H is not localized

(F) When setting compile-time-lexicalized hash %^H one should set the 
corresponding bit of $^H as well.

=item constant(%s): %s

(F) Compile-time-substitutions (such as overloaded constants and
character names) were not correctly set up.

=item defined(@array) is deprecated

(D) defined() is not usually useful on arrays because it checks for an
undefined I<scalar> value.  If you want to see if the array is empty,
just use C<if (@array) { # not empty }> for example.  

=item defined(%hash) is deprecated

(D) defined() is not usually useful on hashes because it checks for an
undefined I<scalar> value.  If you want to see if the hash is empty,
just use C<if (%hash) { # not empty }> for example.  

=item Did not produce a valid header

See Server error.

=item Document contains no data

See Server error.

=item entering effective %s failed

(F) While under the C<use filetest> pragma, switching the real and
effective uids or gids failed.

=item false [] range "%s" in regexp

(W) A character class range must start and end at a literal character, not
another character class like C<\d> or C<[:alpha:]>.  The "-" in your false
range is interpreted as a literal "-".  Consider quoting the "-",  "\-".
See L<perlre>.

=item Filehandle %s opened only for output

(W) You tried to read from a filehandle opened only for writing.  If you
intended it to be a read/write filehandle, you needed to open it with
"+E<lt>" or "+E<gt>" or "+E<gt>E<gt>" instead of with "E<lt>" or nothing.  If
you intended only to read from the file, use "E<lt>".  See
L<perlfunc/open>.

=item Hexadecimal number > 0xffffffff non-portable

(W) The hexadecimal number you specified is larger than 2**32-1
(4294967295) and therefore non-portable between systems.  See
L<perlport> for more on portability concerns.

=item Ill-formed CRTL environ value "%s"

(W) A warning peculiar to VMS.  Perl tried to read the CRTL's internal
environ array, and encountered an element without the C<=> delimiter
used to spearate keys from values.  The element is ignored.

=item Ill-formed message in prime_env_iter: |%s|

(W) A warning peculiar to VMS.  Perl tried to read a logical name
or CLI symbol definition when preparing to iterate over %ENV, and
didn't see the expected delimiter between key and value, so the
line was ignored.

=item Illegal binary digit %s

(F) You used a digit other than 0 or 1 in a binary number.

=item Illegal binary digit %s ignored

(W) You may have tried to use a digit other than 0 or 1 in a binary number.
Interpretation of the binary number stopped before the offending digit.

=item Illegal number of bits in vec

(F) The number of bits in vec() (the third argument) must be a power of
two from 1 to 32 (or 64, if your platform supports that).

=item Integer overflow in %s number

(W) The hexadecimal, octal or binary number you have specified either
as a literal or as an argument to hex() or oct() is too big for your
architecture, and has been converted to a floating point number.  On a
32-bit architecture the largest hexadecimal, octal or binary number
representable without overflow is 0xFFFFFFFF, 037777777777, or
0b11111111111111111111111111111111 respectively.  Note that Perl
transparently promotes all numbers to a floating point representation
internally--subject to loss of precision errors in subsequent
operations.

=item Invalid %s attribute: %s

The indicated attribute for a subroutine or variable was not recognized
by Perl or by a user-supplied handler.  See L<attributes>.

=item Invalid %s attributes: %s

The indicated attributes for a subroutine or variable were not recognized
by Perl or by a user-supplied handler.  See L<attributes>.

=item invalid [] range "%s" in regexp

The offending range is now explicitly displayed.

=item Invalid separator character %s in attribute list

(F) Something other than a comma or whitespace was seen between the
elements of an attribute list.  If the previous attribute
had a parenthesised parameter list, perhaps that list was terminated
too soon.  See L<attributes>.

=item Invalid separator character %s in subroutine attribute list

(F) Something other than a comma or whitespace was seen between the
elements of a subroutine attribute list.  If the previous attribute
had a parenthesised parameter list, perhaps that list was terminated
too soon.

=item leaving effective %s failed

(F) While under the C<use filetest> pragma, switching the real and
effective uids or gids failed.

=item Lvalue subs returning %s not implemented yet

(F) Due to limitations in the current implementation, array and hash
values cannot be returned in subroutines used in lvalue context.
See L<perlsub/"Lvalue subroutines">.

=item Method %s not permitted

See Server error.

=item Missing %sbrace%s on \N{}

(F) Wrong syntax of character name literal C<\N{charname}> within
double-quotish context.

=item Missing command in piped open

(W) You used the C<open(FH, "| command")> or C<open(FH, "command |")>
construction, but the command was missing or blank.

=item Missing name in "my sub"

(F) The reserved syntax for lexically scoped subroutines requires that they
have a name with which they can be found.

=item no UTC offset information; assuming local time is UTC

(S) A warning peculiar to VMS.  Perl was unable to find the local
timezone offset, so it's assuming that local system time is equivalent
to UTC.  If it's not, define the logical name F<SYS$TIMEZONE_DIFFERENTIAL>
to translate to the number of seconds which need to be added to UTC to
get local time.

=item Octal number > 037777777777 non-portable

(W) The octal number you specified is larger than 2**32-1 (4294967295)
and therefore non-portable between systems.  See L<perlport> for more
on portability concerns.

See also L<perlport> for writing portable code.

=item panic: del_backref

(P) Failed an internal consistency check while trying to reset a weak
reference.

=item panic: kid popen errno read

(F) forked child returned an incomprehensible message about its errno.

=item panic: magic_killbackrefs

(P) Failed an internal consistency check while trying to reset all weak
references to an object.

=item Possible Y2K bug: %s

(W) You are concatenating the number 19 with another number, which
could be a potential Year 2000 problem.

=item Premature end of script headers

See Server error.

=item Repeat count in pack overflows

(F) You can't specify a repeat count so large that it overflows
your signed integers.  See L<perlfunc/pack>.

=item Repeat count in unpack overflows

(F) You can't specify a repeat count so large that it overflows
your signed integers.  See L<perlfunc/unpack>.

=item realloc() of freed memory ignored

(S) An internal routine called realloc() on something that had already
been freed.

=item Reference is already weak

(W) You have attempted to weaken a reference that is already weak.
Doing so has no effect.

=item setpgrp can't take arguments

(F) Your system has the setpgrp() from BSD 4.2, which takes no arguments,
unlike POSIX setpgid(), which takes a process ID and process group ID.

=item Strange *+?{} on zero-length expression

(W) You applied a regular expression quantifier in a place where it
makes no sense, such as on a zero-width assertion.
Try putting the quantifier inside the assertion instead.  For example,
the way to match "abc" provided that it is followed by three
repetitions of "xyz" is C</abc(?=(?:xyz){3})/>, not C</abc(?=xyz){3}/>.

=item switching effective %s is not implemented

(F) While under the C<use filetest> pragma, we cannot switch the
real and effective uids or gids.

=item This Perl can't reset CRTL environ elements (%s)

=item This Perl can't set CRTL environ elements (%s=%s)

(W) Warnings peculiar to VMS.  You tried to change or delete an element
of the CRTL's internal environ array, but your copy of Perl wasn't
built with a CRTL that contained the setenv() function.  You'll need to
rebuild Perl with a CRTL that does, or redefine F<PERL_ENV_TABLES> (see
L<perlvms>) so that the environ array isn't the target of the change to
%ENV which produced the warning.

=item Unknown open() mode '%s'

(F) The second argument of 3-argument open() is not among the list
of valid modes: C<L<lt>>, C<L<gt>>, C<E<gt>E<gt>>, C<+L<lt>>,
C<+L<gt>>, C<+E<gt>E<gt>>, C<-|>, C<|->.

=item Unknown process %x sent message to prime_env_iter: %s

(P) An error peculiar to VMS.  Perl was reading values for %ENV before
iterating over it, and someone else stuck a message in the stream of
data Perl expected.  Someone's very confused, or perhaps trying to
subvert Perl's population of %ENV for nefarious purposes.

=item Unrecognized escape \\%c passed through

(W) You used a backslash-character combination which is not recognized
by Perl.  The character was understood literally.

=item Unterminated attribute parameter in attribute list

(F) The lexer saw an opening (left) parenthesis character while parsing an
attribute list, but the matching closing (right) parenthesis
character was not found.  You may need to add (or remove) a backslash
character to get your parentheses to balance.  See L<attributes>.

=item Unterminated attribute list

(F) The lexer found something other than a simple identifier at the start
of an attribute, and it wasn't a semicolon or the start of a
block.  Perhaps you terminated the parameter list of the previous attribute
too soon.  See L<attributes>.

=item Unterminated attribute parameter in subroutine attribute list

(F) The lexer saw an opening (left) parenthesis character while parsing a
subroutine attribute list, but the matching closing (right) parenthesis
character was not found.  You may need to add (or remove) a backslash
character to get your parentheses to balance.

=item Unterminated subroutine attribute list

(F) The lexer found something other than a simple identifier at the start
of a subroutine attribute, and it wasn't a semicolon or the start of a
block.  Perhaps you terminated the parameter list of the previous attribute
too soon.

=item Value of CLI symbol "%s" too long

(W) A warning peculiar to VMS.  Perl tried to read the value of an %ENV
element from a CLI symbol table, and found a resultant string longer
than 1024 characters.  The return value has been truncated to 1024
characters.

=item Version number must be a constant number

(P) The attempt to translate a C<use Module n.n LIST> statement into
its equivalent C<BEGIN> block found an internal inconsistency with
the version number.

=back

=head1 Obsolete Diagnostics

=over 4

=item Character class syntax [: :] is reserved for future extensions

(W) Within regular expression character classes ([]) the syntax beginning
with "[:" and ending with ":]" is reserved for future extensions.
If you need to represent those character sequences inside a regular
expression character class, just quote the square brackets with the
backslash: "\[:" and ":\]".

=item Ill-formed logical name |%s| in prime_env_iter

(W) A warning peculiar to VMS.  A logical name was encountered when preparing
to iterate over %ENV which violates the syntactic rules governing logical
names.  Because it cannot be translated normally, it is skipped, and will not
appear in %ENV.  This may be a benign occurrence, as some software packages
might directly modify logical name tables and introduce nonstandard names,
or it may indicate that a logical name table has been corrupted.

=item regexp too big

(F) The current implementation of regular expressions uses shorts as
address offsets within a string.  Unfortunately this means that if
the regular expression compiles to longer than 32767, it'll blow up.
Usually when you want a regular expression this big, there is a better
way to do it with multiple statements.  See L<perlre>.

=item Use of "$$<digit>" to mean "${$}<digit>" is deprecated

(D) Perl versions before 5.004 misinterpreted any type marker followed
by "$" and a digit.  For example, "$$0" was incorrectly taken to mean
"${$}0" instead of "${$0}".  This bug is (mostly) fixed in Perl 5.004.

However, the developers of Perl 5.004 could not fix this bug completely,
because at least two widely-used modules depend on the old meaning of
"$$0" in a string.  So Perl 5.004 still interprets "$$<digit>" in the
old (broken) way inside strings; but it generates this message as a
warning.  And in Perl 5.005, this special treatment will cease.

=back

=head1 BUGS

If you find what you think is a bug, you might check the
articles recently posted to the comp.lang.perl.misc newsgroup.
There may also be information at http://www.perl.com/perl/, the Perl
Home Page.

If you believe you have an unreported bug, please run the B<perlbug>
program included with your release.  Make sure to trim your bug down
to a tiny but sufficient test case.  Your bug report, along with the
output of C<perl -V>, will be sent off to perlbug@perl.com to be
analysed by the Perl porting team.

=head1 SEE ALSO

The F<Changes> file for exhaustive details on what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

=head1 HISTORY

Written by Gurusamy Sarathy <F<gsar@activestate.com>>, with many
contributions from The Perl Porters.

Send omissions or corrections to <F<perlbug@perl.com>>.

=cut