3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This aliases all the short names to the long
17 names in the current package. Some even have medium names, generally
18 borrowed from B<awk>. In general, it's best to use the
20 use English '-no_match_vars';
22 invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23 a certain performance hit with the use of regular expressions. See
26 Variables that depend on the currently selected filehandle may be set by
27 calling an appropriate object method on the IO::Handle object, although
28 this is less efficient than using the regular built-in variables. (Summary
29 lines below for this contain the word HANDLE.) First you must say
33 after which you may use either
41 Each method returns the old value of the IO::Handle attribute.
42 The methods each take an optional EXPR, which, if supplied, specifies the
43 new value for the IO::Handle attribute in question. If not supplied,
44 most methods do nothing to the current value--except for
45 autoflush(), which will assume a 1 for you, just to be different.
47 Because loading in the IO::Handle class is an expensive operation, you should
48 learn how to use the regular built-in variables.
50 A few of these variables are considered "read-only". This means that if
51 you try to assign to this variable, either directly or indirectly through
52 a reference, you'll raise a run-time exception.
54 You should be very careful when modifying the default values of most
55 special variables described in this document. In most cases you want
56 to localize these variables before changing them, since if you don't,
57 the change may affect other modules which rely on the default values
58 of the special variables that you have changed. This is one of the
59 correct ways to read the whole file at once:
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
66 But the following code is quite bad:
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
73 since some other module, may want to read data from some file in the
74 default "line mode", so if the code we have just presented has been
75 executed, the global value of C<$/> is now changed for any other code
76 running inside the same Perl interpreter.
78 Usually when a variable is localized you want to make sure that this
79 change affects the shortest scope possible. So unless you are already
80 inside some short C<{}> block, you should create one yourself. For
84 open my $fh, "foo" or die $!;
91 Here is an example of how your own code can go broken:
99 # do something with $_
102 You probably expect this code to print:
110 Why? Because nasty_break() modifies C<$_> without localizing it
111 first. The fix is to add local():
115 It's easy to notice the problem in such a short example, but in more
116 complicated code you are looking for trouble if you don't localize
117 changes to the special variables.
119 The following list is ordered by scalar variables first, then the
120 arrays, then the hashes.
129 The default input and pattern-searching space. The following pairs are
132 while (<>) {...} # equivalent only in while!
133 while (defined($_ = <>)) {...}
144 Here are the places where Perl will assume $_ even if you
151 Various unary functions, including functions like ord() and int(), as well
152 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
157 Various list functions like print() and unlink().
161 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
162 without an C<=~> operator.
166 The default iterator variable in a C<foreach> loop if no other
167 variable is supplied.
171 The implicit iterator variable in the grep() and map() functions.
175 The default place to put an input record when a C<< <FH> >>
176 operation's result is tested by itself as the sole criterion of a C<while>
177 test. Outside a C<while> test, this will not happen.
181 As C<$_> is a global variable, this may lead in some cases to unwanted
182 side-effects. As of perl 5.9.1, you can now use a lexical version of
183 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
184 declaring C<our $> restores the global C<$_> in the current scope.
186 (Mnemonic: underline is understood in certain operations.)
197 Special package variables when using sort(), see L<perlfunc/sort>.
198 Because of this specialness $a and $b don't need to be declared
199 (using use vars, or our()) even when using the C<strict 'vars'> pragma.
200 Don't lexicalize them with C<my $a> or C<my $b> if you want to be
201 able to use them in the sort() comparison block or function.
210 Contains the subpattern from the corresponding set of capturing
211 parentheses from the last pattern match, not counting patterns
212 matched in nested blocks that have been exited already. (Mnemonic:
213 like \digits.) These variables are all read-only and dynamically
214 scoped to the current BLOCK.
221 The string matched by the last successful pattern match (not counting
222 any matches hidden within a BLOCK or eval() enclosed by the current
223 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
224 and dynamically scoped to the current BLOCK.
226 The use of this variable anywhere in a program imposes a considerable
227 performance penalty on all regular expression matches. See L</BUGS>.
229 See L</@-> for a replacement.
236 The string preceding whatever was matched by the last successful
237 pattern match (not counting any matches hidden within a BLOCK or eval
238 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
239 string.) This variable is read-only.
241 The use of this variable anywhere in a program imposes a considerable
242 performance penalty on all regular expression matches. See L</BUGS>.
244 See L</@-> for a replacement.
251 The string following whatever was matched by the last successful
252 pattern match (not counting any matches hidden within a BLOCK or eval()
253 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
256 local $_ = 'abcdefghi';
258 print "$`:$&:$'\n"; # prints abc:def:ghi
260 This variable is read-only and dynamically scoped to the current BLOCK.
262 The use of this variable anywhere in a program imposes a considerable
263 performance penalty on all regular expression matches. See L</BUGS>.
265 See L</@-> for a replacement.
267 =item $LAST_PAREN_MATCH
270 X<$+> X<$LAST_PAREN_MATCH>
272 The text matched by the last bracket of the last successful search pattern.
273 This is useful if you don't know which one of a set of alternative patterns
274 matched. For example:
276 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
278 (Mnemonic: be positive and forward looking.)
279 This variable is read-only and dynamically scoped to the current BLOCK.
284 The text matched by the used group most-recently closed (i.e. the group
285 with the rightmost closing parenthesis) of the last successful search
286 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
289 This is primarily used inside C<(?{...})> blocks for examining text
290 recently matched. For example, to effectively capture text to a variable
291 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
293 (?:(...)(?{ $var = $^N }))
295 By setting and then using C<$var> in this way relieves you from having to
296 worry about exactly which numbered set of parentheses they are.
298 This variable is dynamically scoped to the current BLOCK.
300 =item @LAST_MATCH_END
303 X<@+> X<@LAST_MATCH_END>
305 This array holds the offsets of the ends of the last successful
306 submatches in the currently active dynamic scope. C<$+[0]> is
307 the offset into the string of the end of the entire match. This
308 is the same value as what the C<pos> function returns when called
309 on the variable that was matched against. The I<n>th element
310 of this array holds the offset of the I<n>th submatch, so
311 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
312 past where $2 ends, and so on. You can use C<$#+> to determine
313 how many subgroups were in the last successful match. See the
314 examples given for the C<@-> variable.
319 Similar to C<@+>, the C<%+> hash allows access to the named capture
320 buffers, should they exist, in the last successful match in the
321 currently active dynamic scope.
323 C<$+{foo}> is equivalent to C<$1> after the following match:
325 'foo'=~/(?<foo>foo)/;
327 The underlying behaviour of %+ is provided by the L<re::Tie::Hash::NamedCapture>
330 B<Note:> As C<%-> and C<%+> are tied views into a common internal hash
331 associated with the last successful regular expression. Therefore mixing
332 iterative access to them via C<each> may have unpredictable results.
333 Likewise, if the last successful match changes then the results may be
336 =item HANDLE->input_line_number(EXPR)
338 =item $INPUT_LINE_NUMBER
343 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
345 Current line number for the last filehandle accessed.
347 Each filehandle in Perl counts the number of lines that have been read
348 from it. (Depending on the value of C<$/>, Perl's idea of what
349 constitutes a line may not match yours.) When a line is read from a
350 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
351 called on it, C<$.> becomes an alias to the line counter for that
354 You can adjust the counter by assigning to C<$.>, but this will not
355 actually move the seek pointer. I<Localizing C<$.> will not localize
356 the filehandle's line count>. Instead, it will localize perl's notion
357 of which filehandle C<$.> is currently aliased to.
359 C<$.> is reset when the filehandle is closed, but B<not> when an open
360 filehandle is reopened without an intervening close(). For more
361 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
362 an explicit close, line numbers increase across ARGV files (but see
363 examples in L<perlfunc/eof>).
365 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
366 line counter for a given filehandle without having to worry about
367 which handle you last accessed.
369 (Mnemonic: many programs use "." to mean the current line number.)
371 =item IO::Handle->input_record_separator(EXPR)
373 =item $INPUT_RECORD_SEPARATOR
378 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
380 The input record separator, newline by default. This
381 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
382 variable, including treating empty lines as a terminator if set to
383 the null string. (An empty line cannot contain any spaces
384 or tabs.) You may set it to a multi-character string to match a
385 multi-character terminator, or to C<undef> to read through the end
386 of file. Setting it to C<"\n\n"> means something slightly
387 different than setting to C<"">, if the file contains consecutive
388 empty lines. Setting to C<""> will treat two or more consecutive
389 empty lines as a single empty line. Setting to C<"\n\n"> will
390 blindly assume that the next input character belongs to the next
391 paragraph, even if it's a newline. (Mnemonic: / delimits
392 line boundaries when quoting poetry.)
394 local $/; # enable "slurp" mode
395 local $_ = <FH>; # whole file now here
398 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
399 better for something. :-)
401 Setting C<$/> to a reference to an integer, scalar containing an integer, or
402 scalar that's convertible to an integer will attempt to read records
403 instead of lines, with the maximum record size being the referenced
406 local $/ = \32768; # or \"32768", or \$var_containing_32768
407 open my $fh, $myfile or die $!;
410 will read a record of no more than 32768 bytes from FILE. If you're
411 not reading from a record-oriented file (or your OS doesn't have
412 record-oriented files), then you'll likely get a full chunk of data
413 with every read. If a record is larger than the record size you've
414 set, you'll get the record back in pieces. Trying to set the record
415 size to zero or less will cause reading in the (rest of the) whole file.
417 On VMS, record reads are done with the equivalent of C<sysread>,
418 so it's best not to mix record and non-record reads on the same
419 file. (This is unlikely to be a problem, because any file you'd
420 want to read in record mode is probably unusable in line mode.)
421 Non-VMS systems do normal I/O, so it's safe to mix record and
422 non-record reads of a file.
424 See also L<perlport/"Newlines">. Also see C<$.>.
426 =item HANDLE->autoflush(EXPR)
428 =item $OUTPUT_AUTOFLUSH
431 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
433 If set to nonzero, forces a flush right away and after every write
434 or print on the currently selected output channel. Default is 0
435 (regardless of whether the channel is really buffered by the
436 system or not; C<$|> tells you only whether you've asked Perl
437 explicitly to flush after each write). STDOUT will
438 typically be line buffered if output is to the terminal and block
439 buffered otherwise. Setting this variable is useful primarily when
440 you are outputting to a pipe or socket, such as when you are running
441 a Perl program under B<rsh> and want to see the output as it's
442 happening. This has no effect on input buffering. See L<perlfunc/getc>
443 for that. (Mnemonic: when you want your pipes to be piping hot.)
445 =item IO::Handle->output_field_separator EXPR
447 =item $OUTPUT_FIELD_SEPARATOR
452 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
454 The output field separator for the print operator. If defined, this
455 value is printed between each of print's arguments. Default is C<undef>.
456 (Mnemonic: what is printed when there is a "," in your print statement.)
458 =item IO::Handle->output_record_separator EXPR
460 =item $OUTPUT_RECORD_SEPARATOR
465 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
467 The output record separator for the print operator. If defined, this
468 value is printed after the last of print's arguments. Default is C<undef>.
469 (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
470 Also, it's just like C<$/>, but it's what you get "back" from Perl.)
472 =item $LIST_SEPARATOR
475 X<$"> X<$LIST_SEPARATOR>
477 This is like C<$,> except that it applies to array and slice values
478 interpolated into a double-quoted string (or similar interpreted
479 string). Default is a space. (Mnemonic: obvious, I think.)
481 =item $SUBSCRIPT_SEPARATOR
486 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
488 The subscript separator for multidimensional array emulation. If you
489 refer to a hash element as
495 $foo{join($;, $a, $b, $c)}
499 @foo{$a,$b,$c} # a slice--note the @
503 ($foo{$a},$foo{$b},$foo{$c})
505 Default is "\034", the same as SUBSEP in B<awk>. If your
506 keys contain binary data there might not be any safe value for C<$;>.
507 (Mnemonic: comma (the syntactic subscript separator) is a
508 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
509 taken for something more important.)
511 Consider using "real" multidimensional arrays as described
514 =item HANDLE->format_page_number(EXPR)
516 =item $FORMAT_PAGE_NUMBER
519 X<$%> X<$FORMAT_PAGE_NUMBER>
521 The current page number of the currently selected output channel.
523 (Mnemonic: % is page number in B<nroff>.)
525 =item HANDLE->format_lines_per_page(EXPR)
527 =item $FORMAT_LINES_PER_PAGE
530 X<$=> X<$FORMAT_LINES_PER_PAGE>
532 The current page length (printable lines) of the currently selected
533 output channel. Default is 60.
535 (Mnemonic: = has horizontal lines.)
537 =item HANDLE->format_lines_left(EXPR)
539 =item $FORMAT_LINES_LEFT
542 X<$-> X<$FORMAT_LINES_LEFT>
544 The number of lines left on the page of the currently selected output
547 (Mnemonic: lines_on_page - lines_printed.)
549 =item @LAST_MATCH_START
552 X<@-> X<@LAST_MATCH_START>
554 $-[0] is the offset of the start of the last successful match.
555 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
556 I<n>-th subpattern, or undef if the subpattern did not match.
558 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
559 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
560 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
561 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
562 matched subgroup in the last successful match. Contrast with
563 C<$#+>, the number of subgroups in the regular expression. Compare
566 This array holds the offsets of the beginnings of the last
567 successful submatches in the currently active dynamic scope.
568 C<$-[0]> is the offset into the string of the beginning of the
569 entire match. The I<n>th element of this array holds the offset
570 of the I<n>th submatch, so C<$-[1]> is the offset where $1
571 begins, C<$-[2]> the offset where $2 begins, and so on.
573 After a match against some variable $var:
577 =item C<$`> is the same as C<substr($var, 0, $-[0])>
579 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
581 =item C<$'> is the same as C<substr($var, $+[0])>
583 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
585 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
587 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
594 Similar to %+, this variable allows access to the named capture
595 buffers that were defined in the last successful match. It returns
596 a reference to an array containing one value per buffer of a given
599 if ('1234'=~/(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
600 foreach my $name (sort keys(%-)) {
602 foreach my $idx (0..$#$ary) {
603 print "\$-{$name}[$idx] : ",
604 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
617 The behaviour of %- is implemented via the L<re::Tie::Hash::NamedCapture> module.
619 Note that C<%-> and C<%+> are tied views into a common internal hash
620 associated with the last successful regular expression. Therefore mixing
621 iterative access to them via C<each> may have unpredictable results.
622 Likewise, if the last successful match changes then the results may be
625 =item HANDLE->format_name(EXPR)
630 X<$~> X<$FORMAT_NAME>
632 The name of the current report format for the currently selected output
633 channel. Default is the name of the filehandle. (Mnemonic: brother to
636 =item HANDLE->format_top_name(EXPR)
638 =item $FORMAT_TOP_NAME
641 X<$^> X<$FORMAT_TOP_NAME>
643 The name of the current top-of-page format for the currently selected
644 output channel. Default is the name of the filehandle with _TOP
645 appended. (Mnemonic: points to top of page.)
647 =item IO::Handle->format_line_break_characters EXPR
649 =item $FORMAT_LINE_BREAK_CHARACTERS
652 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
654 The current set of characters after which a string may be broken to
655 fill continuation fields (starting with ^) in a format. Default is
656 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
657 poetry is a part of a line.)
659 =item IO::Handle->format_formfeed EXPR
661 =item $FORMAT_FORMFEED
664 X<$^L> X<$FORMAT_FORMFEED>
666 What formats output as a form feed. Default is \f.
671 X<$^A> X<$ACCUMULATOR>
673 The current value of the write() accumulator for format() lines. A format
674 contains formline() calls that put their result into C<$^A>. After
675 calling its format, write() prints out the contents of C<$^A> and empties.
676 So you never really see the contents of C<$^A> unless you call
677 formline() yourself and then look at it. See L<perlform> and
678 L<perlfunc/formline()>.
683 X<$?> X<$CHILD_ERROR>
685 The status returned by the last pipe close, backtick (C<``>) command,
686 successful call to wait() or waitpid(), or from the system()
687 operator. This is just the 16-bit status word returned by the
688 traditional Unix wait() system call (or else is made up to look like it). Thus, the
689 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
690 C<$? & 127> gives which signal, if any, the process died from, and
691 C<$? & 128> reports whether there was a core dump. (Mnemonic:
692 similar to B<sh> and B<ksh>.)
694 Additionally, if the C<h_errno> variable is supported in C, its value
695 is returned via $? if any C<gethost*()> function fails.
697 If you have installed a signal handler for C<SIGCHLD>, the
698 value of C<$?> will usually be wrong outside that handler.
700 Inside an C<END> subroutine C<$?> contains the value that is going to be
701 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
702 change the exit status of your program. For example:
705 $? = 1 if $? == 255; # die would make it 255
708 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
709 actual VMS exit status, instead of the default emulation of POSIX
710 status; see L<perlvms/$?> for details.
712 Also see L<Error Indicators>.
714 =item ${^CHILD_ERROR_NATIVE}
715 X<$^CHILD_ERROR_NATIVE>
717 The native status returned by the last pipe close, backtick (C<``>)
718 command, successful call to wait() or waitpid(), or from the system()
719 operator. On POSIX-like systems this value can be decoded with the
720 WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
721 and WIFCONTINUED functions provided by the L<POSIX> module.
723 Under VMS this reflects the actual VMS exit status; i.e. it is the same
724 as $? when the pragma C<use vmsish 'status'> is in effect.
729 The I<object reference> to the Encode object that is used to convert
730 the source code to Unicode. Thanks to this variable your perl script
731 does not have to be written in UTF-8. Default is I<undef>. The direct
732 manipulation of this variable is highly discouraged. See L<encoding>
740 X<$!> X<$ERRNO> X<$OS_ERROR>
742 If used numerically, yields the current value of the C C<errno>
743 variable, or in other words, if a system or library call fails, it
744 sets this variable. This means that the value of C<$!> is meaningful
745 only I<immediately> after a B<failure>:
747 if (open(FH, $filename)) {
748 # Here $! is meaningless.
751 # ONLY here is $! meaningful.
753 # Already here $! might be meaningless.
755 # Since here we might have either success or failure,
756 # here $! is meaningless.
758 In the above I<meaningless> stands for anything: zero, non-zero,
759 C<undef>. A successful system or library call does B<not> set
760 the variable to zero.
762 If used as a string, yields the corresponding system error string.
763 You can assign a number to C<$!> to set I<errno> if, for instance,
764 you want C<"$!"> to return the string for error I<n>, or you want
765 to set the exit value for the die() operator. (Mnemonic: What just
768 Also see L<Error Indicators>.
773 Each element of C<%!> has a true value only if C<$!> is set to that
774 value. For example, C<$!{ENOENT}> is true if and only if the current
775 value of C<$!> is C<ENOENT>; that is, if the most recent error was
776 "No such file or directory" (or its moral equivalent: not all operating
777 systems give that exact error, and certainly not all languages).
778 To check if a particular key is meaningful on your system, use
779 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
780 See L<Errno> for more information, and also see above for the
783 =item $EXTENDED_OS_ERROR
786 X<$^E> X<$EXTENDED_OS_ERROR>
788 Error information specific to the current operating system. At
789 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
790 (and for MacPerl). On all other platforms, C<$^E> is always just
793 Under VMS, C<$^E> provides the VMS status value from the last
794 system error. This is more specific information about the last
795 system error than that provided by C<$!>. This is particularly
796 important when C<$!> is set to B<EVMSERR>.
798 Under OS/2, C<$^E> is set to the error code of the last call to
799 OS/2 API either via CRT, or directly from perl.
801 Under Win32, C<$^E> always returns the last error information
802 reported by the Win32 call C<GetLastError()> which describes
803 the last error from within the Win32 API. Most Win32-specific
804 code will report errors via C<$^E>. ANSI C and Unix-like calls
805 set C<errno> and so most portable Perl code will report errors
808 Caveats mentioned in the description of C<$!> generally apply to
809 C<$^E>, also. (Mnemonic: Extra error explanation.)
811 Also see L<Error Indicators>.
818 The Perl syntax error message from the last eval() operator.
819 If $@ is the null string, the last eval() parsed and executed
820 correctly (although the operations you invoked may have failed in the
821 normal fashion). (Mnemonic: Where was the syntax error "at"?)
823 Warning messages are not collected in this variable. You can,
824 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
827 Also see L<Error Indicators>.
834 X<$$> X<$PID> X<$PROCESS_ID>
836 The process number of the Perl running this script. You should
837 consider this variable read-only, although it will be altered
838 across fork() calls. (Mnemonic: same as shells.)
840 Note for Linux users: on Linux, the C functions C<getpid()> and
841 C<getppid()> return different values from different threads. In order to
842 be portable, this behavior is not reflected by C<$$>, whose value remains
843 consistent across threads. If you want to call the underlying C<getpid()>,
844 you may use the CPAN module C<Linux::Pid>.
851 X<< $< >> X<$UID> X<$REAL_USER_ID>
853 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
854 if you're running setuid.) You can change both the real uid and
855 the effective uid at the same time by using POSIX::setuid(). Since
856 changes to $< require a system call, check $! after a change attempt to
857 detect any possible errors.
859 =item $EFFECTIVE_USER_ID
864 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
866 The effective uid of this process. Example:
868 $< = $>; # set real to effective uid
869 ($<,$>) = ($>,$<); # swap real and effective uid
871 You can change both the effective uid and the real uid at the same
872 time by using POSIX::setuid(). Changes to $> require a check to $!
873 to detect any possible errors after an attempted change.
875 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
876 C<< $< >> and C<< $> >> can be swapped only on machines
877 supporting setreuid().
884 X<$(> X<$GID> X<$REAL_GROUP_ID>
886 The real gid of this process. If you are on a machine that supports
887 membership in multiple groups simultaneously, gives a space separated
888 list of groups you are in. The first number is the one returned by
889 getgid(), and the subsequent ones by getgroups(), one of which may be
890 the same as the first number.
892 However, a value assigned to C<$(> must be a single number used to
893 set the real gid. So the value given by C<$(> should I<not> be assigned
894 back to C<$(> without being forced numeric, such as by adding zero.
896 You can change both the real gid and the effective gid at the same
897 time by using POSIX::setgid(). Changes to $( require a check to $!
898 to detect any possible errors after an attempted change.
900 (Mnemonic: parentheses are used to I<group> things. The real gid is the
901 group you I<left>, if you're running setgid.)
903 =item $EFFECTIVE_GROUP_ID
908 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
910 The effective gid of this process. If you are on a machine that
911 supports membership in multiple groups simultaneously, gives a space
912 separated list of groups you are in. The first number is the one
913 returned by getegid(), and the subsequent ones by getgroups(), one of
914 which may be the same as the first number.
916 Similarly, a value assigned to C<$)> must also be a space-separated
917 list of numbers. The first number sets the effective gid, and
918 the rest (if any) are passed to setgroups(). To get the effect of an
919 empty list for setgroups(), just repeat the new effective gid; that is,
920 to force an effective gid of 5 and an effectively empty setgroups()
921 list, say C< $) = "5 5" >.
923 You can change both the effective gid and the real gid at the same
924 time by using POSIX::setgid() (use only a single numeric argument).
925 Changes to $) require a check to $! to detect any possible errors
926 after an attempted change.
928 (Mnemonic: parentheses are used to I<group> things. The effective gid
929 is the group that's I<right> for you, if you're running setgid.)
931 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
932 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
933 and C<$)> can be swapped only on machines supporting setregid().
938 X<$0> X<$PROGRAM_NAME>
940 Contains the name of the program being executed.
942 On some (read: not all) operating systems assigning to C<$0> modifies
943 the argument area that the C<ps> program sees. On some platforms you
944 may have to use special C<ps> options or a different C<ps> to see the
945 changes. Modifying the $0 is more useful as a way of indicating the
946 current program state than it is for hiding the program you're
947 running. (Mnemonic: same as B<sh> and B<ksh>.)
949 Note that there are platform specific limitations on the maximum
950 length of C<$0>. In the most extreme case it may be limited to the
951 space occupied by the original C<$0>.
953 In some platforms there may be arbitrary amount of padding, for
954 example space characters, after the modified name as shown by C<ps>.
955 In some platforms this padding may extend all the way to the original
956 length of the argument area, no matter what you do (this is the case
957 for example with Linux 2.2).
959 Note for BSD users: setting C<$0> does not completely remove "perl"
960 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
961 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
962 and the " (perl)" suffix are shown depends on your exact BSD variant
963 and version). This is an operating system feature, Perl cannot help it.
965 In multithreaded scripts Perl coordinates the threads so that any
966 thread may modify its copy of the C<$0> and the change becomes visible
967 to ps(1) (assuming the operating system plays along). Note that
968 the view of C<$0> the other threads have will not change since they
969 have their own copies of it.
974 The index of the first element in an array, and of the first character
975 in a substring. Default is 0, but you could theoretically set it
976 to 1 to make Perl behave more like B<awk> (or Fortran) when
977 subscripting and when evaluating the index() and substr() functions.
978 (Mnemonic: [ begins subscripts.)
980 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
981 directive, and cannot influence the behavior of any other file.
982 (That's why you can only assign compile-time constants to it.)
983 Its use is highly discouraged.
985 Note that, unlike other compile-time directives (such as L<strict>),
986 assignment to C<$[> can be seen from outer lexical scopes in the same file.
987 However, you can use local() on it to strictly bind its value to a
993 The version + patchlevel / 1000 of the Perl interpreter. This variable
994 can be used to determine whether the Perl interpreter executing a
995 script is in the right range of versions. (Mnemonic: Is this version
996 of perl in the right bracket?) Example:
998 warn "No checksumming!\n" if $] < 3.019;
1000 See also the documentation of C<use VERSION> and C<require VERSION>
1001 for a convenient way to fail if the running Perl interpreter is too old.
1003 The floating point representation can sometimes lead to inaccurate
1004 numeric comparisons. See C<$^V> for a more modern representation of
1005 the Perl version that allows accurate string comparisons.
1010 X<$^C> X<$COMPILING>
1012 The current value of the flag associated with the B<-c> switch.
1013 Mainly of use with B<-MO=...> to allow code to alter its behavior
1014 when being compiled, such as for example to AUTOLOAD at compile
1015 time rather than normal, deferred loading. See L<perlcc>. Setting
1016 C<$^C = 1> is similar to calling C<B::minus_c>.
1021 X<$^D> X<$DEBUGGING>
1023 The current value of the debugging flags. (Mnemonic: value of B<-D>
1024 switch.) May be read or set. Like its command-line equivalent, you can use
1025 numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
1027 =item ${^RE_DEBUG_FLAGS}
1029 The current value of the regex debugging flags. Set to 0 for no debug output
1030 even when the re 'debug' module is loaded. See L<re> for details.
1032 =item ${^RE_TRIE_MAXBUF}
1034 Controls how certain regex optimisations are applied and how much memory they
1035 utilize. This value by default is 65536 which corresponds to a 512kB temporary
1036 cache. Set this to a higher value to trade memory for speed when matching
1037 large alternations. Set it to a lower value if you want the optimisations to
1038 be as conservative of memory as possible but still occur, and set it to a
1039 negative value to prevent the optimisation and conserve the most memory.
1040 Under normal situations this variable should be of no interest to you.
1042 =item $SYSTEM_FD_MAX
1045 X<$^F> X<$SYSTEM_FD_MAX>
1047 The maximum system file descriptor, ordinarily 2. System file
1048 descriptors are passed to exec()ed processes, while higher file
1049 descriptors are not. Also, during an open(), system file descriptors are
1050 preserved even if the open() fails. (Ordinary file descriptors are
1051 closed before the open() is attempted.) The close-on-exec
1052 status of a file descriptor will be decided according to the value of
1053 C<$^F> when the corresponding file, pipe, or socket was opened, not the
1058 WARNING: This variable is strictly for internal use only. Its availability,
1059 behavior, and contents are subject to change without notice.
1061 This variable contains compile-time hints for the Perl interpreter. At the
1062 end of compilation of a BLOCK the value of this variable is restored to the
1063 value when the interpreter started to compile the BLOCK.
1065 When perl begins to parse any block construct that provides a lexical scope
1066 (e.g., eval body, required file, subroutine body, loop body, or conditional
1067 block), the existing value of $^H is saved, but its value is left unchanged.
1068 When the compilation of the block is completed, it regains the saved value.
1069 Between the points where its value is saved and restored, code that
1070 executes within BEGIN blocks is free to change the value of $^H.
1072 This behavior provides the semantic of lexical scoping, and is used in,
1073 for instance, the C<use strict> pragma.
1075 The contents should be an integer; different bits of it are used for
1076 different pragmatic flags. Here's an example:
1078 sub add_100 { $^H |= 0x100 }
1085 Consider what happens during execution of the BEGIN block. At this point
1086 the BEGIN block has already been compiled, but the body of foo() is still
1087 being compiled. The new value of $^H will therefore be visible only while
1088 the body of foo() is being compiled.
1090 Substitution of the above BEGIN block with:
1092 BEGIN { require strict; strict->import('vars') }
1094 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1095 version of the same lexical pragma:
1097 BEGIN { require strict; strict->import('vars') if $condition }
1101 The %^H hash provides the same scoping semantic as $^H. This makes it
1102 useful for implementation of lexically scoped pragmas. See L<perlpragma>.
1107 X<$^I> X<$INPLACE_EDIT>
1109 The current value of the inplace-edit extension. Use C<undef> to disable
1110 inplace editing. (Mnemonic: value of B<-i> switch.)
1115 By default, running out of memory is an untrappable, fatal error.
1116 However, if suitably built, Perl can use the contents of C<$^M>
1117 as an emergency memory pool after die()ing. Suppose that your Perl
1118 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
1121 $^M = 'a' x (1 << 16);
1123 would allocate a 64K buffer for use in an emergency. See the
1124 F<INSTALL> file in the Perl distribution for information on how to
1125 add custom C compilation flags when compiling perl. To discourage casual
1126 use of this advanced feature, there is no L<English|English> long name for
1134 The name of the operating system under which this copy of Perl was
1135 built, as determined during the configuration process. The value
1136 is identical to C<$Config{'osname'}>. See also L<Config> and the
1137 B<-V> command-line switch documented in L<perlrun>.
1139 In Windows platforms, $^O is not very helpful: since it is always
1140 C<MSWin32>, it doesn't tell the difference between
1141 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1142 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1143 between the variants.
1147 An internal variable used by PerlIO. A string in two parts, separated
1148 by a C<\0> byte, the first part describes the input layers, the second
1149 part describes the output layers.
1156 The internal variable for debugging support. The meanings of the
1157 various bits are subject to change, but currently indicate:
1163 Debug subroutine enter/exit.
1167 Line-by-line debugging.
1171 Switch off optimizations.
1175 Preserve more data for future interactive inspections.
1179 Keep info about source lines on which a subroutine is defined.
1183 Start with single-step on.
1187 Use subroutine address instead of name when reporting.
1191 Report C<goto &subroutine> as well.
1195 Provide informative "file" names for evals based on the place they were compiled.
1199 Provide informative names to anonymous subroutines based on the place they
1204 Debug assertion subroutines enter/exit.
1208 Some bits may be relevant at compile-time only, some at
1209 run-time only. This is a new mechanism and the details may change.
1211 =item $LAST_REGEXP_CODE_RESULT
1214 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1216 The result of evaluation of the last successful C<(?{ code })>
1217 regular expression assertion (see L<perlre>). May be written to.
1219 =item $EXCEPTIONS_BEING_CAUGHT
1222 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1224 Current state of the interpreter.
1227 --------- -------------------
1228 undef Parsing module/eval
1229 true (1) Executing an eval
1232 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
1239 The time at which the program began running, in seconds since the
1240 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1241 and B<-C> filetests are based on this value.
1245 Reflects if taint mode is on or off. 1 for on (the program was run with
1246 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
1247 B<-t> or B<-TU>). This variable is read-only.
1251 Reflects certain Unicode settings of Perl. See L<perlrun>
1252 documentation for the C<-C> switch for more information about
1253 the possible values. This variable is set during Perl startup
1254 and is thereafter read-only.
1258 This variable controls the state of the internal UTF-8 offset caching code.
1259 1 for on (the default), 0 for off, -1 to debug the caching code by checking
1260 all its results against linear scans, and panicking on any discrepancy.
1262 =item ${^UTF8LOCALE}
1264 This variable indicates whether an UTF-8 locale was detected by perl at
1265 startup. This information is used by perl when it's in
1266 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1267 switch); see L<perlrun> for more info on this.
1272 X<$^V> X<$PERL_VERSION>
1274 The revision, version, and subversion of the Perl interpreter, represented
1275 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
1276 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1277 C<$^V eq v5.6.0>. Note that the characters in this string value can
1278 potentially be in Unicode range.
1280 This variable first appeared in perl 5.6.0; earlier versions of perl will
1281 see an undefined value.
1283 This can be used to determine whether the Perl interpreter executing a
1284 script is in the right range of versions. (Mnemonic: use ^V for Version
1287 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
1289 To convert C<$^V> into its string representation use sprintf()'s
1290 C<"%vd"> conversion:
1292 printf "version is v%vd\n", $^V; # Perl's version
1294 See the documentation of C<use VERSION> and C<require VERSION>
1295 for a convenient way to fail if the running Perl interpreter is too old.
1297 See also C<$]> for an older representation of the Perl version.
1304 The current value of the warning switch, initially true if B<-w>
1305 was used, false otherwise, but directly modifiable. (Mnemonic:
1306 related to the B<-w> switch.) See also L<warnings>.
1308 =item ${^WARNING_BITS}
1310 The current set of warning checks enabled by the C<use warnings> pragma.
1311 See the documentation of C<warnings> for more details.
1313 =item ${^WIN32_SLOPPY_STAT}
1315 If this variable is set to a true value, then stat() on Windows will
1316 not try to open the file. This means that the link count cannot be
1317 determined and file attributes may be out of date if additional
1318 hardlinks to the file exist. On the other hand, not opening the file
1319 is considerably faster, especially for files on network drives.
1321 This variable could be set in the F<sitecustomize.pl> file to
1322 configure the local Perl installation to use "sloppy" stat() by
1323 default. See L<perlrun> for more information about site
1326 =item $EXECUTABLE_NAME
1329 X<$^X> X<$EXECUTABLE_NAME>
1331 The name used to execute the current copy of Perl, from C's
1332 C<argv[0]> or (where supported) F</proc/self/exe>.
1334 Depending on the host operating system, the value of $^X may be
1335 a relative or absolute pathname of the perl program file, or may
1336 be the string used to invoke perl but not the pathname of the
1337 perl program file. Also, most operating systems permit invoking
1338 programs that are not in the PATH environment variable, so there
1339 is no guarantee that the value of $^X is in PATH. For VMS, the
1340 value may or may not include a version number.
1342 You usually can use the value of $^X to re-invoke an independent
1343 copy of the same perl that is currently running, e.g.,
1345 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1347 But recall that not all operating systems support forking or
1348 capturing of the output of commands, so this complex statement
1349 may not be portable.
1351 It is not safe to use the value of $^X as a path name of a file,
1352 as some operating systems that have a mandatory suffix on
1353 executable files do not require use of the suffix when invoking
1354 a command. To convert the value of $^X to a path name, use the
1355 following statements:
1357 # Build up a set of file names (not command names).
1361 {$this_perl .= $Config{_exe}
1362 unless $this_perl =~ m/$Config{_exe}$/i;}
1364 Because many operating systems permit anyone with read access to
1365 the Perl program file to make a copy of it, patch the copy, and
1366 then execute the copy, the security-conscious Perl programmer
1367 should take care to invoke the installed copy of perl, not the
1368 copy referenced by $^X. The following statements accomplish
1369 this goal, and produce a pathname that can be invoked as a
1370 command or referenced as a file.
1373 $secure_perl_path = $Config{perlpath};
1375 {$secure_perl_path .= $Config{_exe}
1376 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
1381 The special filehandle that iterates over command-line filenames in
1382 C<@ARGV>. Usually written as the null filehandle in the angle operator
1383 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1384 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1385 corresponding to the last file opened by C<< <> >>. In particular,
1386 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1387 may not cause your function to automatically read the contents of all the
1393 contains the name of the current file when reading from <>.
1398 The array @ARGV contains the command-line arguments intended for
1399 the script. C<$#ARGV> is generally the number of arguments minus
1400 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1401 command name itself. See C<$0> for the command name.
1406 The special filehandle that points to the currently open output file
1407 when doing edit-in-place processing with B<-i>. Useful when you have
1408 to do a lot of inserting and don't want to keep modifying $_. See
1409 L<perlrun> for the B<-i> switch.
1414 The array @F contains the fields of each line read in when autosplit
1415 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1416 is package-specific, and must be declared or given a full package name
1417 if not in package main when running under C<strict 'vars'>.
1422 The array @INC contains the list of places that the C<do EXPR>,
1423 C<require>, or C<use> constructs look for their library files. It
1424 initially consists of the arguments to any B<-I> command-line
1425 switches, followed by the default Perl library, probably
1426 F</usr/local/lib/perl>, followed by ".", to represent the current
1427 directory. ("." will not be appended if taint checks are enabled, either by
1428 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
1429 the C<use lib> pragma to get the machine-dependent library properly
1432 use lib '/mypath/libdir/';
1435 You can also insert hooks into the file inclusion system by putting Perl
1436 code directly into @INC. Those hooks may be subroutine references, array
1437 references or blessed objects. See L<perlfunc/require> for details.
1444 Within a subroutine the array @_ contains the parameters passed to that
1445 subroutine. See L<perlsub>.
1450 The hash %INC contains entries for each filename included via the
1451 C<do>, C<require>, or C<use> operators. The key is the filename
1452 you specified (with module names converted to pathnames), and the
1453 value is the location of the file found. The C<require>
1454 operator uses this hash to determine whether a particular file has
1455 already been included.
1457 If the file was loaded via a hook (e.g. a subroutine reference, see
1458 L<perlfunc/require> for a description of these hooks), this hook is
1459 by default inserted into %INC in place of a filename. Note, however,
1460 that the hook may have set the %INC entry by itself to provide some more
1468 The hash %ENV contains your current environment. Setting a
1469 value in C<ENV> changes the environment for any child processes
1470 you subsequently fork() off.
1477 The hash C<%SIG> contains signal handlers for signals. For example:
1479 sub handler { # 1st argument is signal name
1481 print "Caught a SIG$sig--shutting down\n";
1486 $SIG{'INT'} = \&handler;
1487 $SIG{'QUIT'} = \&handler;
1489 $SIG{'INT'} = 'DEFAULT'; # restore default action
1490 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1492 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1493 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1496 Here are some other examples:
1498 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1499 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1500 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1501 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1503 Be sure not to use a bareword as the name of a signal handler,
1504 lest you inadvertently call it.
1506 If your system has the sigaction() function then signal handlers are
1507 installed using it. This means you get reliable signal handling.
1509 The default delivery policy of signals changed in Perl 5.8.0 from
1510 immediate (also known as "unsafe") to deferred, also known as
1511 "safe signals". See L<perlipc> for more information.
1513 Certain internal hooks can be also set using the %SIG hash. The
1514 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1515 about to be printed. The warning message is passed as the first
1516 argument. The presence of a C<__WARN__> hook causes the ordinary printing
1517 of warnings to C<STDERR> to be suppressed. You can use this to save warnings
1518 in a variable, or turn warnings into fatal errors, like this:
1520 local $SIG{__WARN__} = sub { die $_[0] };
1523 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
1524 disable warnings using the empty subroutine:
1526 local $SIG{__WARN__} = sub {};
1528 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1529 is about to be thrown. The error message is passed as the first
1530 argument. When a C<__DIE__> hook routine returns, the exception
1531 processing continues as it would have in the absence of the hook,
1532 unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>.
1533 The C<__DIE__> handler is explicitly disabled during the call, so that you
1534 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1536 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1537 even inside an eval(). Do not use this to rewrite a pending exception
1538 in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>.
1539 This strange action at a distance may be fixed in a future release
1540 so that C<$SIG{__DIE__}> is only called if your program is about
1541 to exit, as was the original intent. Any other use is deprecated.
1543 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1544 they may be called to report (probable) errors found by the parser.
1545 In such a case the parser may be in inconsistent state, so any
1546 attempt to evaluate Perl code from such a handler will probably
1547 result in a segfault. This means that warnings or errors that
1548 result from parsing Perl should be used with extreme caution, like
1551 require Carp if defined $^S;
1552 Carp::confess("Something wrong") if defined &Carp::confess;
1553 die "Something wrong, but could not load Carp to give backtrace...
1554 To see backtrace try starting Perl with -MCarp switch";
1556 Here the first line will load Carp I<unless> it is the parser who
1557 called the handler. The second line will print backtrace and die if
1558 Carp was available. The third line will be executed only if Carp was
1561 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1562 L<warnings> for additional information.
1566 =head2 Error Indicators
1567 X<error> X<exception>
1569 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1570 about different types of error conditions that may appear during
1571 execution of a Perl program. The variables are shown ordered by
1572 the "distance" between the subsystem which reported the error and
1573 the Perl process. They correspond to errors detected by the Perl
1574 interpreter, C library, operating system, or an external program,
1577 To illustrate the differences between these variables, consider the
1578 following Perl expression, which uses a single-quoted string:
1581 open my $pipe, "/cdrom/install |" or die $!;
1583 close $pipe or die "bad pipe: $?, $!";
1586 After execution of this statement all 4 variables may have been set.
1588 C<$@> is set if the string to be C<eval>-ed did not compile (this
1589 may happen if C<open> or C<close> were imported with bad prototypes),
1590 or if Perl code executed during evaluation die()d . In these cases
1591 the value of $@ is the compile error, or the argument to C<die>
1592 (which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
1595 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1596 and C<close> are translated to calls in the C run-time library and
1597 thence to the operating system kernel. C<$!> is set to the C library's
1598 C<errno> if one of these calls fails.
1600 Under a few operating systems, C<$^E> may contain a more verbose
1601 error indicator, such as in this case, "CDROM tray not closed."
1602 Systems that do not support extended error messages leave C<$^E>
1605 Finally, C<$?> may be set to non-0 value if the external program
1606 F</cdrom/install> fails. The upper eight bits reflect specific
1607 error conditions encountered by the program (the program's exit()
1608 value). The lower eight bits reflect mode of failure, like signal
1609 death and core dump information See wait(2) for details. In
1610 contrast to C<$!> and C<$^E>, which are set only if error condition
1611 is detected, the variable C<$?> is set on each C<wait> or pipe
1612 C<close>, overwriting the old value. This is more like C<$@>, which
1613 on every eval() is always set on failure and cleared on success.
1615 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1618 =head2 Technical Note on the Syntax of Variable Names
1620 Variable names in Perl can have several formats. Usually, they
1621 must begin with a letter or underscore, in which case they can be
1622 arbitrarily long (up to an internal limit of 251 characters) and
1623 may contain letters, digits, underscores, or the special sequence
1624 C<::> or C<'>. In this case, the part before the last C<::> or
1625 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1627 Perl variable names may also be a sequence of digits or a single
1628 punctuation or control character. These names are all reserved for
1629 special uses by Perl; for example, the all-digits names are used
1630 to hold data captured by backreferences after a regular expression
1631 match. Perl has a special syntax for the single-control-character
1632 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1633 character. For example, the notation C<$^W> (dollar-sign caret
1634 C<W>) is the scalar variable whose name is the single character
1635 control-C<W>. This is better than typing a literal control-C<W>
1638 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1639 strings that begin with control characters (or better yet, a caret).
1640 These variables must be written in the form C<${^Foo}>; the braces
1641 are not optional. C<${^Foo}> denotes the scalar variable whose
1642 name is a control-C<F> followed by two C<o>'s. These variables are
1643 reserved for future special uses by Perl, except for the ones that
1644 begin with C<^_> (control-underscore or caret-underscore). No
1645 control-character name that begins with C<^_> will acquire a special
1646 meaning in any future version of Perl; such names may therefore be
1647 used safely in programs. C<$^_> itself, however, I<is> reserved.
1649 Perl identifiers that begin with digits, control characters, or
1650 punctuation characters are exempt from the effects of the C<package>
1651 declaration and are always forced to be in package C<main>; they are
1652 also exempt from C<strict 'vars'> errors. A few other names are also
1653 exempt in these ways:
1661 In particular, the new special C<${^_XYZ}> variables are always taken
1662 to be in package C<main>, regardless of any C<package> declarations
1667 Due to an unfortunate accident of Perl's implementation, C<use
1668 English> imposes a considerable performance penalty on all regular
1669 expression matches in a program, regardless of whether they occur
1670 in the scope of C<use English>. For that reason, saying C<use
1671 English> in libraries is strongly discouraged. See the
1672 Devel::SawAmpersand module documentation from CPAN
1673 ( http://www.cpan.org/modules/by-module/Devel/ )
1674 for more information. Writing C<use English '-no_match_vars';>
1675 avoids the performance penalty.
1677 Having to even think about the C<$^S> variable in your exception
1678 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1679 invites grievous and difficult to track down errors. Avoid it
1680 and use an C<END{}> or CORE::GLOBAL::die override instead.