3 perlvar - Perl predefined variables
7 =head2 The Syntax of Variable Names
9 Variable names in Perl can have several formats. Usually, they
10 must begin with a letter or underscore, in which case they can be
11 arbitrarily long (up to an internal limit of 251 characters) and
12 may contain letters, digits, underscores, or the special sequence
13 C<::> or C<'>. In this case, the part before the last C<::> or
14 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
16 Perl variable names may also be a sequence of digits or a single
17 punctuation or control character. These names are all reserved for
18 special uses by Perl; for example, the all-digits names are used
19 to hold data captured by backreferences after a regular expression
20 match. Perl has a special syntax for the single-control-character
21 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
22 character. For example, the notation C<$^W> (dollar-sign caret
23 C<W>) is the scalar variable whose name is the single character
24 control-C<W>. This is better than typing a literal control-C<W>
27 Since Perl 5.6, Perl variable names may be alphanumeric
28 strings that begin with control characters (or better yet, a caret).
29 These variables must be written in the form C<${^Foo}>; the braces
30 are not optional. C<${^Foo}> denotes the scalar variable whose
31 name is a control-C<F> followed by two C<o>'s. These variables are
32 reserved for future special uses by Perl, except for the ones that
33 begin with C<^_> (control-underscore or caret-underscore). No
34 control-character name that begins with C<^_> will acquire a special
35 meaning in any future version of Perl; such names may therefore be
36 used safely in programs. C<$^_> itself, however, I<is> reserved.
38 Perl identifiers that begin with digits, control characters, or
39 punctuation characters are exempt from the effects of the C<package>
40 declaration and are always forced to be in package C<main>; they are
41 also exempt from C<strict 'vars'> errors. A few other names are also
50 In particular, the special C<${^_XYZ}> variables are always taken
51 to be in package C<main>, regardless of any C<package> declarations
54 =head1 SPECIAL VARIABLES
56 The following names have special meaning to Perl. Most punctuation
57 names have reasonable mnemonics, or analogs in the shells.
58 Nevertheless, if you wish to use long variable names, you need only say:
62 at the top of your program. This aliases all the short names to the long
63 names in the current package. Some even have medium names, generally
64 borrowed from B<awk>. To avoid a performance hit, if you don't need the
65 C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
68 use English '-no_match_vars';
70 Before you continue, note the sort order for variables. In general, we
71 first list the variables in case-insensitive, almost-lexigraphical
72 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
73 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
74 For variables with the same identifier, we list it in order of scalar,
75 array, hash, and bareword.
77 =head2 General Variables
86 The default input and pattern-searching space. The following pairs are
89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
101 Here are the places where Perl will assume C<$_> even if you don't use it:
107 The following functions use C<$_> as a default argument:
109 abs, alarm, chomp, chop, chr, chroot,
110 cos, defined, eval, evalbytes, exp, glob,
111 hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
112 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
113 rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
118 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
123 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
124 when used without an C<=~> operator.
128 The default iterator variable in a C<foreach> loop if no other
129 variable is supplied.
133 The implicit iterator variable in the C<grep()> and C<map()> functions.
137 The implicit variable of C<given()>.
141 The default place to put an input record when a C<< <FH> >>
142 operation's result is tested by itself as the sole criterion of a C<while>
143 test. Outside a C<while> test, this will not happen.
147 As C<$_> is a global variable, this may lead in some cases to unwanted
148 side-effects. As of perl 5.10, you can now use a lexical version of
149 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
150 declaring C<our $_> restores the global C<$_> in the current scope.
152 Mnemonic: underline is understood in certain operations.
159 Within a subroutine the array C<@_> contains the parameters passed to
160 that subroutine. Inside a subroutine, C<@_> is the default array for
161 the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
165 =item $LIST_SEPARATOR
168 X<$"> X<$LIST_SEPARATOR>
170 When an array or an array slice is interpolated into a double-quoted
171 string or a similar context such as C</.../>, its elements are
172 separated by this value. Default is a space. For example, this:
174 print "The array is: @array\n";
176 is equivalent to this:
178 print "The array is: " . join($", @array) . "\n";
180 Mnemonic: works in double-quoted context.
187 X<$$> X<$PID> X<$PROCESS_ID>
189 The process number of the Perl running this script. Though you I<can> set
190 this variable, doing so is generally discouraged, although it can be
191 invaluable for some testing purposes. It will be reset automatically
192 across C<fork()> calls.
194 Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
195 would emulate POSIX semantics on Linux systems using LinuxThreads, a
196 partial implementation of POSIX Threads that has since been superseded
197 by the Native POSIX Thread Library (NPTL).
199 LinuxThreads is now obsolete on Linux, and caching C<getpid()>
200 like this made embedding perl unnecessarily complex (since you'd have
201 to manually update the value of $$), so now C<$$> and C<getppid()>
202 will always return the same values as the underlying C library.
204 Debian GNU/kFreeBSD systems also used LinuxThreads up until and
205 including the 6.0 release, but after that moved to FreeBSD thread
206 semantics, which are POSIX-like.
208 To see if your system is affected by this discrepancy check if
209 C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
210 value. NTPL threads preserve the POSIX semantics.
212 Mnemonic: same as shells.
217 X<$0> X<$PROGRAM_NAME>
219 Contains the name of the program being executed.
221 On some (but not all) operating systems assigning to C<$0> modifies
222 the argument area that the C<ps> program sees. On some platforms you
223 may have to use special C<ps> options or a different C<ps> to see the
224 changes. Modifying the C<$0> is more useful as a way of indicating the
225 current program state than it is for hiding the program you're
228 Note that there are platform-specific limitations on the maximum
229 length of C<$0>. In the most extreme case it may be limited to the
230 space occupied by the original C<$0>.
232 In some platforms there may be arbitrary amount of padding, for
233 example space characters, after the modified name as shown by C<ps>.
234 In some platforms this padding may extend all the way to the original
235 length of the argument area, no matter what you do (this is the case
236 for example with Linux 2.2).
238 Note for BSD users: setting C<$0> does not completely remove "perl"
239 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
240 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
241 and the " (perl)" suffix are shown depends on your exact BSD variant
242 and version). This is an operating system feature, Perl cannot help it.
244 In multithreaded scripts Perl coordinates the threads so that any
245 thread may modify its copy of the C<$0> and the change becomes visible
246 to ps(1) (assuming the operating system plays along). Note that
247 the view of C<$0> the other threads have will not change since they
248 have their own copies of it.
250 If the program has been given to perl via the switches C<-e> or C<-E>,
251 C<$0> will contain the string C<"-e">.
253 On Linux as of perl 5.14 the legacy process name will be set with
254 C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
255 perl has done since version 4.000. Now system utilities that read the
256 legacy process name such as ps, top and killall will recognize the
257 name you set when assigning to C<$0>. The string you supply will be
258 cut off at 16 bytes, this is a limitation imposed by Linux.
260 Mnemonic: same as B<sh> and B<ksh>.
267 X<$(> X<$GID> X<$REAL_GROUP_ID>
269 The real gid of this process. If you are on a machine that supports
270 membership in multiple groups simultaneously, gives a space separated
271 list of groups you are in. The first number is the one returned by
272 C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
273 the same as the first number.
275 However, a value assigned to C<$(> must be a single number used to
276 set the real gid. So the value given by C<$(> should I<not> be assigned
277 back to C<$(> without being forced numeric, such as by adding zero. Note
278 that this is different to the effective gid (C<$)>) which does take a
281 You can change both the real gid and the effective gid at the same
282 time by using C<POSIX::setgid()>. Changes
283 to C<$(> require a check to C<$!>
284 to detect any possible errors after an attempted change.
286 Mnemonic: parentheses are used to I<group> things. The real gid is the
287 group you I<left>, if you're running setgid.
289 =item $EFFECTIVE_GROUP_ID
294 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
296 The effective gid of this process. If you are on a machine that
297 supports membership in multiple groups simultaneously, gives a space
298 separated list of groups you are in. The first number is the one
299 returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
300 one of which may be the same as the first number.
302 Similarly, a value assigned to C<$)> must also be a space-separated
303 list of numbers. The first number sets the effective gid, and
304 the rest (if any) are passed to C<setgroups()>. To get the effect of an
305 empty list for C<setgroups()>, just repeat the new effective gid; that is,
306 to force an effective gid of 5 and an effectively empty C<setgroups()>
307 list, say C< $) = "5 5" >.
309 You can change both the effective gid and the real gid at the same
310 time by using C<POSIX::setgid()> (use only a single numeric argument).
311 Changes to C<$)> require a check to C<$!> to detect any possible errors
312 after an attempted change.
314 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
315 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
316 and C<$)> can be swapped only on machines supporting C<setregid()>.
318 Mnemonic: parentheses are used to I<group> things. The effective gid
319 is the group that's I<right> for you, if you're running setgid.
326 X<< $< >> X<$UID> X<$REAL_USER_ID>
328 The real uid of this process. You can change both the real uid and the
329 effective uid at the same time by using C<POSIX::setuid()>. Since
330 changes to C<< $< >> require a system call, check C<$!> after a change
331 attempt to detect any possible errors.
333 Mnemonic: it's the uid you came I<from>, if you're running setuid.
335 =item $EFFECTIVE_USER_ID
340 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
342 The effective uid of this process. For example:
344 $< = $>; # set real to effective uid
345 ($<,$>) = ($>,$<); # swap real and effective uids
347 You can change both the effective uid and the real uid at the same
348 time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
349 to C<$!> to detect any possible errors after an attempted change.
351 C<< $< >> and C<< $> >> can be swapped only on machines
352 supporting C<setreuid()>.
354 Mnemonic: it's the uid you went I<to>, if you're running setuid.
356 =item $SUBSCRIPT_SEPARATOR
361 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
363 The subscript separator for multidimensional array emulation. If you
364 refer to a hash element as
370 $foo{join($;, $a, $b, $c)}
374 @foo{$a,$b,$c} # a slice--note the @
378 ($foo{$a},$foo{$b},$foo{$c})
380 Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
381 binary data there might not be any safe value for C<$;>.
383 Consider using "real" multidimensional arrays as described
386 Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
393 Special package variables when using C<sort()>, see L<perlfunc/sort>.
394 Because of this specialness C<$a> and C<$b> don't need to be declared
395 (using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
396 pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
397 be able to use them in the C<sort()> comparison block or function.
402 The hash C<%ENV> contains your current environment. Setting a
403 value in C<ENV> changes the environment for any child processes
404 you subsequently C<fork()> off.
409 X<$^F> X<$SYSTEM_FD_MAX>
411 The maximum system file descriptor, ordinarily 2. System file
412 descriptors are passed to C<exec()>ed processes, while higher file
413 descriptors are not. Also, during an
414 C<open()>, system file descriptors are
415 preserved even if the C<open()> fails (ordinary file descriptors are
416 closed before the C<open()> is attempted). The close-on-exec
417 status of a file descriptor will be decided according to the value of
418 C<$^F> when the corresponding file, pipe, or socket was opened, not the
419 time of the C<exec()>.
424 The array C<@F> contains the fields of each line read in when autosplit
425 mode is turned on. See L<perlrun> for the B<-a> switch. This array
426 is package-specific, and must be declared or given a full package name
427 if not in package main when running under C<strict 'vars'>.
432 The array C<@INC> contains the list of places that the C<do EXPR>,
433 C<require>, or C<use> constructs look for their library files. It
434 initially consists of the arguments to any B<-I> command-line
435 switches, followed by the default Perl library, probably
436 F</usr/local/lib/perl>, followed by ".", to represent the current
437 directory. ("." will not be appended if taint checks are enabled,
438 either by C<-T> or by C<-t>.) If you need to modify this at runtime,
439 you should use the C<use lib> pragma to get the machine-dependent
440 library properly loaded also:
442 use lib '/mypath/libdir/';
445 You can also insert hooks into the file inclusion system by putting Perl
446 code directly into C<@INC>. Those hooks may be subroutine references,
447 array references or blessed objects. See L<perlfunc/require> for details.
452 The hash C<%INC> contains entries for each filename included via the
453 C<do>, C<require>, or C<use> operators. The key is the filename
454 you specified (with module names converted to pathnames), and the
455 value is the location of the file found. The C<require>
456 operator uses this hash to determine whether a particular file has
457 already been included.
459 If the file was loaded via a hook (e.g. a subroutine reference, see
460 L<perlfunc/require> for a description of these hooks), this hook is
461 by default inserted into C<%INC> in place of a filename. Note, however,
462 that the hook may have set the C<%INC> entry by itself to provide some more
468 X<$^I> X<$INPLACE_EDIT>
470 The current value of the inplace-edit extension. Use C<undef> to disable
473 Mnemonic: value of B<-i> switch.
478 By default, running out of memory is an untrappable, fatal error.
479 However, if suitably built, Perl can use the contents of C<$^M>
480 as an emergency memory pool after C<die()>ing. Suppose that your Perl
481 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
484 $^M = 'a' x (1 << 16);
486 would allocate a 64K buffer for use in an emergency. See the
487 F<INSTALL> file in the Perl distribution for information on how to
488 add custom C compilation flags when compiling perl. To discourage casual
489 use of this advanced feature, there is no L<English|English> long name for
492 This variable was added in Perl 5.004.
499 The name of the operating system under which this copy of Perl was
500 built, as determined during the configuration process. For examples
501 see L<perlport/PLATFORMS>.
503 The value is identical to C<$Config{'osname'}>. See also L<Config>
504 and the B<-V> command-line switch documented in L<perlrun>.
506 In Windows platforms, C<$^O> is not very helpful: since it is always
507 C<MSWin32>, it doesn't tell the difference between
508 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
509 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
510 between the variants.
512 This variable was added in Perl 5.003.
517 The hash C<%SIG> contains signal handlers for signals. For example:
519 sub handler { # 1st argument is signal name
521 print "Caught a SIG$sig--shutting down\n";
526 $SIG{'INT'} = \&handler;
527 $SIG{'QUIT'} = \&handler;
529 $SIG{'INT'} = 'DEFAULT'; # restore default action
530 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
532 Using a value of C<'IGNORE'> usually has the effect of ignoring the
533 signal, except for the C<CHLD> signal. See L<perlipc> for more about
536 Here are some other examples:
538 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
540 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
542 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
543 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
546 Be sure not to use a bareword as the name of a signal handler,
547 lest you inadvertently call it.
549 If your system has the C<sigaction()> function then signal handlers
550 are installed using it. This means you get reliable signal handling.
552 The default delivery policy of signals changed in Perl 5.8.0 from
553 immediate (also known as "unsafe") to deferred, also known as "safe
554 signals". See L<perlipc> for more information.
556 Certain internal hooks can be also set using the C<%SIG> hash. The
557 routine indicated by C<$SIG{__WARN__}> is called when a warning
558 message is about to be printed. The warning message is passed as the
559 first argument. The presence of a C<__WARN__> hook causes the
560 ordinary printing of warnings to C<STDERR> to be suppressed. You can
561 use this to save warnings in a variable, or turn warnings into fatal
564 local $SIG{__WARN__} = sub { die $_[0] };
567 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
568 disable warnings using the empty subroutine:
570 local $SIG{__WARN__} = sub {};
572 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
573 exception is about to be thrown. The error message is passed as the
574 first argument. When a C<__DIE__> hook routine returns, the exception
575 processing continues as it would have in the absence of the hook,
576 unless the hook routine itself exits via a C<goto &sub>, a loop exit,
577 or a C<die()>. The C<__DIE__> handler is explicitly disabled during
578 the call, so that you can die from a C<__DIE__> handler. Similarly
581 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
582 even inside an C<eval()>. Do not use this to rewrite a pending
583 exception in C<$@>, or as a bizarre substitute for overriding
584 C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
585 in a future release so that C<$SIG{__DIE__}> is only called if your
586 program is about to exit, as was the original intent. Any other use is
589 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
590 may be called to report (probable) errors found by the parser. In such
591 a case the parser may be in inconsistent state, so any attempt to
592 evaluate Perl code from such a handler will probably result in a
593 segfault. This means that warnings or errors that result from parsing
594 Perl should be used with extreme caution, like this:
596 require Carp if defined $^S;
597 Carp::confess("Something wrong") if defined &Carp::confess;
598 die "Something wrong, but could not load Carp to give "
600 . "To see backtrace try starting Perl with -MCarp switch";
602 Here the first line will load C<Carp> I<unless> it is the parser who
603 called the handler. The second line will print backtrace and die if
604 C<Carp> was available. The third line will be executed only if C<Carp> was
607 Having to even think about the C<$^S> variable in your exception
608 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
609 invites grievous and difficult to track down errors. Avoid it
610 and use an C<END{}> or CORE::GLOBAL::die override instead.
612 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
613 L<warnings> for additional information.
620 The time at which the program began running, in seconds since the
621 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
622 and B<-C> filetests are based on this value.
627 X<$^V> X<$PERL_VERSION>
629 The revision, version, and subversion of the Perl interpreter,
630 represented as a C<version> object.
632 This variable first appeared in perl 5.6.0; earlier versions of perl
633 will see an undefined value. Before perl 5.10.0 C<$^V> was represented
636 C<$^V> can be used to determine whether the Perl interpreter executing
637 a script is in the right range of versions. For example:
639 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
641 To convert C<$^V> into its string representation use C<sprintf()>'s
644 printf "version is v%vd\n", $^V; # Perl's version
646 See the documentation of C<use VERSION> and C<require VERSION>
647 for a convenient way to fail if the running Perl interpreter is too old.
649 See also C<$]> for an older representation of the Perl version.
651 This variable was added in Perl 5.6.
653 Mnemonic: use ^V for Version Control.
655 =item ${^WIN32_SLOPPY_STAT}
656 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
658 If this variable is set to a true value, then C<stat()> on Windows will
659 not try to open the file. This means that the link count cannot be
660 determined and file attributes may be out of date if additional
661 hardlinks to the file exist. On the other hand, not opening the file
662 is considerably faster, especially for files on network drives.
664 This variable could be set in the F<sitecustomize.pl> file to
665 configure the local Perl installation to use "sloppy" C<stat()> by
666 default. See the documentation for B<-f> in
667 L<perlrun|perlrun/"Command Switches"> for more information about site
670 This variable was added in Perl 5.10.
672 =item $EXECUTABLE_NAME
675 X<$^X> X<$EXECUTABLE_NAME>
677 The name used to execute the current copy of Perl, from C's
678 C<argv[0]> or (where supported) F</proc/self/exe>.
680 Depending on the host operating system, the value of C<$^X> may be
681 a relative or absolute pathname of the perl program file, or may
682 be the string used to invoke perl but not the pathname of the
683 perl program file. Also, most operating systems permit invoking
684 programs that are not in the PATH environment variable, so there
685 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
686 value may or may not include a version number.
688 You usually can use the value of C<$^X> to re-invoke an independent
689 copy of the same perl that is currently running, e.g.,
691 @first_run = `$^X -le "print int rand 100 for 1..100"`;
693 But recall that not all operating systems support forking or
694 capturing of the output of commands, so this complex statement
697 It is not safe to use the value of C<$^X> as a path name of a file,
698 as some operating systems that have a mandatory suffix on
699 executable files do not require use of the suffix when invoking
700 a command. To convert the value of C<$^X> to a path name, use the
701 following statements:
703 # Build up a set of file names (not command names).
707 $this_perl .= $Config{_exe}
708 unless $this_perl =~ m/$Config{_exe}$/i;
711 Because many operating systems permit anyone with read access to
712 the Perl program file to make a copy of it, patch the copy, and
713 then execute the copy, the security-conscious Perl programmer
714 should take care to invoke the installed copy of perl, not the
715 copy referenced by C<$^X>. The following statements accomplish
716 this goal, and produce a pathname that can be invoked as a
717 command or referenced as a file.
720 my $secure_perl_path = $Config{perlpath};
722 $secure_perl_path .= $Config{_exe}
723 unless $secure_perl_path =~ m/$Config{_exe}$/i;
728 =head2 Variables related to regular expressions
730 Most of the special variables related to regular expressions are side
731 effects. Perl sets these variables when it has a successful match, so
732 you should check the match result before using them. For instance:
734 if( /P(A)TT(ER)N/ ) {
735 print "I found $1 and $2\n";
738 These variables are read-only and dynamically-scoped, unless we note
741 The dynamic nature of the regular expression variables means that
742 their value is limited to the block that they are in, as demonstrated
745 my $outer = 'Wallace and Grommit';
746 my $inner = 'Mutt and Jeff';
748 my $pattern = qr/(\S+) and (\S+)/;
750 sub show_n { print "\$1 is $1; \$2 is $2\n" }
754 show_n() if $outer =~ m/$pattern/;
757 show_n() if $inner =~ m/$pattern/;
763 The output shows that while in the C<OUTER> block, the values of C<$1>
764 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
765 block, the values of C<$1> and C<$2> are from the match against
766 C<$inner>, but only until the end of the block (i.e. the dynamic
767 scope). After the C<INNER> block completes, the values of C<$1> and
768 C<$2> return to the values for the match against C<$outer> even though
769 we have not made another match:
771 $1 is Wallace; $2 is Grommit
772 $1 is Mutt; $2 is Jeff
773 $1 is Wallace; $2 is Grommit
775 Due to an unfortunate accident of Perl's implementation, C<use
776 English> imposes a considerable performance penalty on all regular
777 expression matches in a program because it uses the C<$`>, C<$&>, and
778 C<$'>, regardless of whether they occur in the scope of C<use
779 English>. For that reason, saying C<use English> in libraries is
780 strongly discouraged unless you import it without the match variables:
782 use English '-no_match_vars'
784 The C<Devel::NYTProf> and C<Devel::FindAmpersand>
785 modules can help you find uses of these
786 problematic match variables in your code.
788 Since Perl 5.10, you can use the C</p> match operator flag and the
789 C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
790 so you only suffer the performance penalties.
794 =item $<I<digits>> ($1, $2, ...)
797 Contains the subpattern from the corresponding set of capturing
798 parentheses from the last successful pattern match, not counting patterns
799 matched in nested blocks that have been exited already.
801 These variables are read-only and dynamically-scoped.
803 Mnemonic: like \digits.
810 The string matched by the last successful pattern match (not counting
811 any matches hidden within a BLOCK or C<eval()> enclosed by the current
814 The use of this variable anywhere in a program imposes a considerable
815 performance penalty on all regular expression matches. To avoid this
816 penalty, you can extract the same substring by using L</@->. Starting
817 with Perl 5.10, you can use the C</p> match flag and the C<${^MATCH}>
818 variable to do the same thing for particular match operations.
820 This variable is read-only and dynamically-scoped.
822 Mnemonic: like C<&> in some editors.
827 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
828 performance penalty associated with that variable, and is only guaranteed
829 to return a defined value when the pattern was compiled or executed with
832 This variable was added in Perl 5.10.
834 This variable is read-only and dynamically-scoped.
839 X<$`> X<$PREMATCH> X<${^PREMATCH}>
841 The string preceding whatever was matched by the last successful
842 pattern match, not counting any matches hidden within a BLOCK or C<eval>
843 enclosed by the current BLOCK.
845 The use of this variable anywhere in a program imposes a considerable
846 performance penalty on all regular expression matches. To avoid this
847 penalty, you can extract the same substring by using L</@->. Starting
848 with Perl 5.10, you can use the C</p> match flag and the
849 C<${^PREMATCH}> variable to do the same thing for particular match
852 This variable is read-only and dynamically-scoped.
854 Mnemonic: C<`> often precedes a quoted string.
857 X<$`> X<${^PREMATCH}>
859 This is similar to C<$`> ($PREMATCH) except that it does not incur the
860 performance penalty associated with that variable, and is only guaranteed
861 to return a defined value when the pattern was compiled or executed with
864 This variable was added in Perl 5.10
866 This variable is read-only and dynamically-scoped.
871 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
873 The string following whatever was matched by the last successful
874 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
875 enclosed by the current BLOCK). Example:
877 local $_ = 'abcdefghi';
879 print "$`:$&:$'\n"; # prints abc:def:ghi
881 The use of this variable anywhere in a program imposes a considerable
882 performance penalty on all regular expression matches.
883 To avoid this penalty, you can extract the same substring by
884 using L</@->. Starting with Perl 5.10, you can use the C</p> match flag
885 and the C<${^POSTMATCH}> variable to do the same thing for particular
888 This variable is read-only and dynamically-scoped.
890 Mnemonic: C<'> often follows a quoted string.
893 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
895 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
896 performance penalty associated with that variable, and is only guaranteed
897 to return a defined value when the pattern was compiled or executed with
900 This variable was added in Perl 5.10.
902 This variable is read-only and dynamically-scoped.
904 =item $LAST_PAREN_MATCH
907 X<$+> X<$LAST_PAREN_MATCH>
909 The text matched by the last bracket of the last successful search pattern.
910 This is useful if you don't know which one of a set of alternative patterns
911 matched. For example:
913 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
915 This variable is read-only and dynamically-scoped.
917 Mnemonic: be positive and forward looking.
919 =item $LAST_SUBMATCH_RESULT
922 X<$^N> X<$LAST_SUBMATCH_RESULT>
924 The text matched by the used group most-recently closed (i.e. the group
925 with the rightmost closing parenthesis) of the last successful search
928 This is primarily used inside C<(?{...})> blocks for examining text
929 recently matched. For example, to effectively capture text to a variable
930 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
932 (?:(...)(?{ $var = $^N }))
934 By setting and then using C<$var> in this way relieves you from having to
935 worry about exactly which numbered set of parentheses they are.
937 This variable was added in Perl 5.8.
939 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
941 =item @LAST_MATCH_END
944 X<@+> X<@LAST_MATCH_END>
946 This array holds the offsets of the ends of the last successful
947 submatches in the currently active dynamic scope. C<$+[0]> is
948 the offset into the string of the end of the entire match. This
949 is the same value as what the C<pos> function returns when called
950 on the variable that was matched against. The I<n>th element
951 of this array holds the offset of the I<n>th submatch, so
952 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
953 past where C<$2> ends, and so on. You can use C<$#+> to determine
954 how many subgroups were in the last successful match. See the
955 examples given for the C<@-> variable.
957 This variable was added in Perl 5.6.
959 =item %LAST_PAREN_MATCH
962 X<%+> X<%LAST_PAREN_MATCH>
964 Similar to C<@+>, the C<%+> hash allows access to the named capture
965 buffers, should they exist, in the last successful match in the
966 currently active dynamic scope.
968 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
970 'foo' =~ /(?<foo>foo)/;
972 The keys of the C<%+> hash list only the names of buffers that have
973 captured (and that are thus associated to defined values).
975 The underlying behaviour of C<%+> is provided by the
976 L<Tie::Hash::NamedCapture> module.
978 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
979 associated with the last successful regular expression. Therefore mixing
980 iterative access to them via C<each> may have unpredictable results.
981 Likewise, if the last successful match changes, then the results may be
984 This variable was added in Perl 5.10.
986 This variable is read-only and dynamically-scoped.
988 =item @LAST_MATCH_START
991 X<@-> X<@LAST_MATCH_START>
993 C<$-[0]> is the offset of the start of the last successful match.
994 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
995 I<n>-th subpattern, or undef if the subpattern did not match.
997 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
998 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
999 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1000 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1001 last matched subgroup in the last successful match. Contrast with
1002 C<$#+>, the number of subgroups in the regular expression. Compare
1005 This array holds the offsets of the beginnings of the last
1006 successful submatches in the currently active dynamic scope.
1007 C<$-[0]> is the offset into the string of the beginning of the
1008 entire match. The I<n>th element of this array holds the offset
1009 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1010 begins, C<$-[2]> the offset where C<$2> begins, and so on.
1012 After a match against some variable C<$var>:
1016 =item C<$`> is the same as C<substr($var, 0, $-[0])>
1018 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1020 =item C<$'> is the same as C<substr($var, $+[0])>
1022 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1024 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1026 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1030 This variable was added in Perl 5.6.
1032 =item %LAST_MATCH_START
1035 X<%-> X<%LAST_MATCH_START>
1037 Similar to C<%+>, this variable allows access to the named capture groups
1038 in the last successful match in the currently active dynamic scope. To
1039 each capture group name found in the regular expression, it associates a
1040 reference to an array containing the list of values captured by all
1041 buffers with that name (should there be several of them), in the order
1046 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1047 foreach my $bufname (sort keys %-) {
1048 my $ary = $-{$bufname};
1049 foreach my $idx (0..$#$ary) {
1050 print "\$-{$bufname}[$idx] : ",
1051 (defined($ary->[$idx])
1066 The keys of the C<%-> hash correspond to all buffer names found in
1067 the regular expression.
1069 The behaviour of C<%-> is implemented via the
1070 L<Tie::Hash::NamedCapture> module.
1072 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1073 associated with the last successful regular expression. Therefore mixing
1074 iterative access to them via C<each> may have unpredictable results.
1075 Likewise, if the last successful match changes, then the results may be
1078 This variable was added in Perl 5.10
1080 This variable is read-only and dynamically-scoped.
1082 =item $LAST_REGEXP_CODE_RESULT
1085 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1087 The result of evaluation of the last successful C<(?{ code })>
1088 regular expression assertion (see L<perlre>). May be written to.
1090 This variable was added in Perl 5.005.
1092 =item ${^RE_DEBUG_FLAGS}
1093 X<${^RE_DEBUG_FLAGS}>
1095 The current value of the regex debugging flags. Set to 0 for no debug output
1096 even when the C<re 'debug'> module is loaded. See L<re> for details.
1098 This variable was added in Perl 5.10.
1100 =item ${^RE_TRIE_MAXBUF}
1101 X<${^RE_TRIE_MAXBUF}>
1103 Controls how certain regex optimisations are applied and how much memory they
1104 utilize. This value by default is 65536 which corresponds to a 512kB
1105 temporary cache. Set this to a higher value to trade
1106 memory for speed when matching large alternations. Set
1107 it to a lower value if you want the optimisations to
1108 be as conservative of memory as possible but still occur, and set it to a
1109 negative value to prevent the optimisation and conserve the most memory.
1110 Under normal situations this variable should be of no interest to you.
1112 This variable was added in Perl 5.10.
1116 =head2 Variables related to filehandles
1118 Variables that depend on the currently selected filehandle may be set
1119 by calling an appropriate object method on the C<IO::Handle> object,
1120 although this is less efficient than using the regular built-in
1121 variables. (Summary lines below for this contain the word HANDLE.)
1126 after which you may use either
1132 HANDLE->method(EXPR)
1134 Each method returns the old value of the C<IO::Handle> attribute. The
1135 methods each take an optional EXPR, which, if supplied, specifies the
1136 new value for the C<IO::Handle> attribute in question. If not
1137 supplied, most methods do nothing to the current value--except for
1138 C<autoflush()>, which will assume a 1 for you, just to be different.
1140 Because loading in the C<IO::Handle> class is an expensive operation,
1141 you should learn how to use the regular built-in variables.
1143 A few of these variables are considered "read-only". This means that
1144 if you try to assign to this variable, either directly or indirectly
1145 through a reference, you'll raise a run-time exception.
1147 You should be very careful when modifying the default values of most
1148 special variables described in this document. In most cases you want
1149 to localize these variables before changing them, since if you don't,
1150 the change may affect other modules which rely on the default values
1151 of the special variables that you have changed. This is one of the
1152 correct ways to read the whole file at once:
1154 open my $fh, "<", "foo" or die $!;
1155 local $/; # enable localized slurp mode
1156 my $content = <$fh>;
1159 But the following code is quite bad:
1161 open my $fh, "<", "foo" or die $!;
1162 undef $/; # enable slurp mode
1163 my $content = <$fh>;
1166 since some other module, may want to read data from some file in the
1167 default "line mode", so if the code we have just presented has been
1168 executed, the global value of C<$/> is now changed for any other code
1169 running inside the same Perl interpreter.
1171 Usually when a variable is localized you want to make sure that this
1172 change affects the shortest scope possible. So unless you are already
1173 inside some short C<{}> block, you should create one yourself. For
1177 open my $fh, "<", "foo" or die $!;
1184 Here is an example of how your own code can go broken:
1194 # do something with $_
1197 You probably expect this code to print the equivalent of
1201 but instead you get:
1205 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1206 first. The value you set in C<nasty_break()> is still there when you
1207 return. The fix is to add C<local()> so the value doesn't leak out of
1212 It's easy to notice the problem in such a short example, but in more
1213 complicated code you are looking for trouble if you don't localize
1214 changes to the special variables.
1221 Contains the name of the current file when reading from C<< <> >>.
1226 The array C<@ARGV> contains the command-line arguments intended for
1227 the script. C<$#ARGV> is generally the number of arguments minus
1228 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1229 command name itself. See L</$0> for the command name.
1234 The special filehandle that iterates over command-line filenames in
1235 C<@ARGV>. Usually written as the null filehandle in the angle operator
1236 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1237 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1238 corresponding to the last file opened by C<< <> >>. In particular,
1239 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1240 may not cause your function to automatically read the contents of all the
1246 The special filehandle that points to the currently open output file
1247 when doing edit-in-place processing with B<-i>. Useful when you have
1248 to do a lot of inserting and don't want to keep modifying C<$_>. See
1249 L<perlrun> for the B<-i> switch.
1251 =item Handle->output_field_separator( EXPR )
1253 =item $OUTPUT_FIELD_SEPARATOR
1258 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1260 The output field separator for the print operator. If defined, this
1261 value is printed between each of print's arguments. Default is C<undef>.
1263 Mnemonic: what is printed when there is a "," in your print statement.
1265 =item HANDLE->input_line_number( EXPR )
1267 =item $INPUT_LINE_NUMBER
1272 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1274 Current line number for the last filehandle accessed.
1276 Each filehandle in Perl counts the number of lines that have been read
1277 from it. (Depending on the value of C<$/>, Perl's idea of what
1278 constitutes a line may not match yours.) When a line is read from a
1279 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1280 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1281 for that filehandle.
1283 You can adjust the counter by assigning to C<$.>, but this will not
1284 actually move the seek pointer. I<Localizing C<$.> will not localize
1285 the filehandle's line count>. Instead, it will localize perl's notion
1286 of which filehandle C<$.> is currently aliased to.
1288 C<$.> is reset when the filehandle is closed, but B<not> when an open
1289 filehandle is reopened without an intervening C<close()>. For more
1290 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1291 an explicit close, line numbers increase across C<ARGV> files (but see
1292 examples in L<perlfunc/eof>).
1294 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1295 line counter for a given filehandle without having to worry about
1296 which handle you last accessed.
1298 Mnemonic: many programs use "." to mean the current line number.
1300 =item HANDLE->input_record_separator( EXPR )
1302 =item $INPUT_RECORD_SEPARATOR
1307 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1309 The input record separator, newline by default. This influences Perl's
1310 idea of what a "line" is. Works like B<awk>'s RS variable, including
1311 treating empty lines as a terminator if set to the null string (an
1312 empty line cannot contain any spaces or tabs). You may set it to a
1313 multi-character string to match a multi-character terminator, or to
1314 C<undef> to read through the end of file. Setting it to C<"\n\n">
1315 means something slightly different than setting to C<"">, if the file
1316 contains consecutive empty lines. Setting to C<""> will treat two or
1317 more consecutive empty lines as a single empty line. Setting to
1318 C<"\n\n"> will blindly assume that the next input character belongs to
1319 the next paragraph, even if it's a newline.
1321 local $/; # enable "slurp" mode
1322 local $_ = <FH>; # whole file now here
1325 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1326 be better for something. :-)
1328 Setting C<$/> to a reference to an integer, scalar containing an
1329 integer, or scalar that's convertible to an integer will attempt to
1330 read records instead of lines, with the maximum record size being the
1331 referenced integer. So this:
1333 local $/ = \32768; # or \"32768", or \$var_containing_32768
1334 open my $fh, "<", $myfile or die $!;
1337 will read a record of no more than 32768 bytes from FILE. If you're
1338 not reading from a record-oriented file (or your OS doesn't have
1339 record-oriented files), then you'll likely get a full chunk of data
1340 with every read. If a record is larger than the record size you've
1341 set, you'll get the record back in pieces. Trying to set the record
1342 size to zero or less will cause reading in the (rest of the) whole file.
1344 On VMS only, record reads bypass PerlIO layers and any associated
1345 buffering,so you must not mix record and non-record reads on the
1346 same filehandle. Record mode mixes with line mode only when the
1347 same buffering layer is in use for both modes.
1349 If you perform a record read on a FILE with an encoding layer such as
1350 C<:encoding(latin1)> or C<:utf8>, you may get an invalid string as a
1351 result, may leave the FILE positioned between characters in the stream
1352 and may not be reading the number of bytes from the underlying file
1353 that you specified. This behaviour may change without warning in a
1354 future version of perl.
1356 See also L<perlport/"Newlines">. Also see L</$.>.
1358 Mnemonic: / delimits line boundaries when quoting poetry.
1360 =item Handle->output_record_separator( EXPR )
1362 =item $OUTPUT_RECORD_SEPARATOR
1367 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1369 The output record separator for the print operator. If defined, this
1370 value is printed after the last of print's arguments. Default is C<undef>.
1372 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1373 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1375 =item HANDLE->autoflush( EXPR )
1377 =item $OUTPUT_AUTOFLUSH
1380 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1382 If set to nonzero, forces a flush right away and after every write or
1383 print on the currently selected output channel. Default is 0
1384 (regardless of whether the channel is really buffered by the system or
1385 not; C<$|> tells you only whether you've asked Perl explicitly to
1386 flush after each write). STDOUT will typically be line buffered if
1387 output is to the terminal and block buffered otherwise. Setting this
1388 variable is useful primarily when you are outputting to a pipe or
1389 socket, such as when you are running a Perl program under B<rsh> and
1390 want to see the output as it's happening. This has no effect on input
1391 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1392 how to select the output channel. See also L<IO::Handle>.
1394 Mnemonic: when you want your pipes to be piping hot.
1398 =head3 Variables related to formats
1400 The special variables for formats are a subset of those for
1401 filehandles. See L<perlform> for more information about Perl's
1409 X<$^A> X<$ACCUMULATOR>
1411 The current value of the C<write()> accumulator for C<format()> lines.
1412 A format contains C<formline()> calls that put their result into
1413 C<$^A>. After calling its format, C<write()> prints out the contents
1414 of C<$^A> and empties. So you never really see the contents of C<$^A>
1415 unless you call C<formline()> yourself and then look at it. See
1416 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1418 =item HANDLE->format_formfeed(EXPR)
1420 =item $FORMAT_FORMFEED
1423 X<$^L> X<$FORMAT_FORMFEED>
1425 What formats output as a form feed. The default is C<\f>.
1427 =item HANDLE->format_page_number(EXPR)
1429 =item $FORMAT_PAGE_NUMBER
1432 X<$%> X<$FORMAT_PAGE_NUMBER>
1434 The current page number of the currently selected output channel.
1436 Mnemonic: C<%> is page number in B<nroff>.
1438 =item HANDLE->format_lines_left(EXPR)
1440 =item $FORMAT_LINES_LEFT
1443 X<$-> X<$FORMAT_LINES_LEFT>
1445 The number of lines left on the page of the currently selected output
1448 Mnemonic: lines_on_page - lines_printed.
1450 =item Handle->format_line_break_characters EXPR
1452 =item $FORMAT_LINE_BREAK_CHARACTERS
1455 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1457 The current set of characters after which a string may be broken to
1458 fill continuation fields (starting with C<^>) in a format. The default is
1459 S<" \n-">, to break on a space, newline, or a hyphen.
1461 Mnemonic: a "colon" in poetry is a part of a line.
1463 =item HANDLE->format_lines_per_page(EXPR)
1465 =item $FORMAT_LINES_PER_PAGE
1468 X<$=> X<$FORMAT_LINES_PER_PAGE>
1470 The current page length (printable lines) of the currently selected
1471 output channel. The default is 60.
1473 Mnemonic: = has horizontal lines.
1475 =item HANDLE->format_top_name(EXPR)
1477 =item $FORMAT_TOP_NAME
1480 X<$^> X<$FORMAT_TOP_NAME>
1482 The name of the current top-of-page format for the currently selected
1483 output channel. The default is the name of the filehandle with C<_TOP>
1484 appended. For example, the default format top name for the C<STDOUT>
1485 filehandle is C<STDOUT_TOP>.
1487 Mnemonic: points to top of page.
1489 =item HANDLE->format_name(EXPR)
1494 X<$~> X<$FORMAT_NAME>
1496 The name of the current report format for the currently selected
1497 output channel. The default format name is the same as the filehandle
1498 name. For example, the default format name for the C<STDOUT>
1499 filehandle is just C<STDOUT>.
1501 Mnemonic: brother to C<$^>.
1505 =head2 Error Variables
1506 X<error> X<exception>
1508 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1509 about different types of error conditions that may appear during
1510 execution of a Perl program. The variables are shown ordered by
1511 the "distance" between the subsystem which reported the error and
1512 the Perl process. They correspond to errors detected by the Perl
1513 interpreter, C library, operating system, or an external program,
1516 To illustrate the differences between these variables, consider the
1517 following Perl expression, which uses a single-quoted string. After
1518 execution of this statement, perl may have set all four special error
1522 open my $pipe, "/cdrom/install |" or die $!;
1524 close $pipe or die "bad pipe: $?, $!";
1527 When perl executes the C<eval()> expression, it translates the
1528 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1529 and thence to the operating system kernel. perl sets C<$!> to
1530 the C library's C<errno> if one of these calls fails.
1532 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1533 happen if C<open> or C<close> were imported with bad prototypes), or
1534 if Perl code executed during evaluation C<die()>d. In these cases the
1535 value of C<$@> is the compile error, or the argument to C<die> (which
1536 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1538 Under a few operating systems, C<$^E> may contain a more verbose error
1539 indicator, such as in this case, "CDROM tray not closed." Systems that
1540 do not support extended error messages leave C<$^E> the same as C<$!>.
1542 Finally, C<$?> may be set to non-0 value if the external program
1543 F</cdrom/install> fails. The upper eight bits reflect specific error
1544 conditions encountered by the program (the program's C<exit()> value).
1545 The lower eight bits reflect mode of failure, like signal death and
1546 core dump information. See L<wait(2)> for details. In contrast to
1547 C<$!> and C<$^E>, which are set only if error condition is detected,
1548 the variable C<$?> is set on each C<wait> or pipe C<close>,
1549 overwriting the old value. This is more like C<$@>, which on every
1550 C<eval()> is always set on failure and cleared on success.
1552 For more details, see the individual descriptions at C<$@>, C<$!>,
1557 =item ${^CHILD_ERROR_NATIVE}
1558 X<$^CHILD_ERROR_NATIVE>
1560 The native status returned by the last pipe close, backtick (C<``>)
1561 command, successful call to C<wait()> or C<waitpid()>, or from the
1562 C<system()> operator. On POSIX-like systems this value can be decoded
1563 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1564 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1566 Under VMS this reflects the actual VMS exit status; i.e. it is the
1567 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1569 This variable was added in Perl 5.8.9.
1571 =item $EXTENDED_OS_ERROR
1574 X<$^E> X<$EXTENDED_OS_ERROR>
1576 Error information specific to the current operating system. At the
1577 moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1578 for MacPerl). On all other platforms, C<$^E> is always just the same
1581 Under VMS, C<$^E> provides the VMS status value from the last system
1582 error. This is more specific information about the last system error
1583 than that provided by C<$!>. This is particularly important when C<$!>
1584 is set to B<EVMSERR>.
1586 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1587 API either via CRT, or directly from perl.
1589 Under Win32, C<$^E> always returns the last error information reported
1590 by the Win32 call C<GetLastError()> which describes the last error
1591 from within the Win32 API. Most Win32-specific code will report errors
1592 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1593 portable Perl code will report errors via C<$!>.
1595 Caveats mentioned in the description of C<$!> generally apply to
1598 This variable was added in Perl 5.003.
1600 Mnemonic: Extra error explanation.
1602 =item $EXCEPTIONS_BEING_CAUGHT
1605 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1607 Current state of the interpreter.
1610 --------- -------------------
1611 undef Parsing module/eval
1612 true (1) Executing an eval
1615 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1618 This variable was added in Perl 5.004.
1625 The current value of the warning switch, initially true if B<-w> was
1626 used, false otherwise, but directly modifiable.
1628 See also L<warnings>.
1630 Mnemonic: related to the B<-w> switch.
1632 =item ${^WARNING_BITS}
1635 The current set of warning checks enabled by the C<use warnings> pragma.
1636 It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1637 values are considered internal to the L<warnings> pragma and may change
1638 between versions of Perl.
1640 This variable was added in Perl 5.6.
1647 X<$!> X<$ERRNO> X<$OS_ERROR>
1649 When referenced, C<$!> retrieves the current value
1650 of the C C<errno> integer variable.
1651 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1652 When referenced as a string, C<$!> yields the system error string
1653 corresponding to C<errno>.
1655 Many system or library calls set C<errno> if they fail,
1656 to indicate the cause of failure. They usually do B<not>
1657 set C<errno> to zero if they succeed. This means C<errno>,
1658 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1660 if (open my $fh, "<", $filename) {
1661 # Here $! is meaningless.
1665 # ONLY here is $! meaningful.
1667 # Already here $! might be meaningless.
1669 # Since here we might have either success or failure,
1670 # $! is meaningless.
1672 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1673 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1674 It can be used immediately before invoking the C<die()> operator,
1675 to set the exit value, or to inspect the system error string
1676 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1678 Mnemonic: What just went bang?
1685 X<%!> X<%OS_ERROR> X<%ERRNO>
1687 Each element of C<%!> has a true value only if C<$!> is set to that
1688 value. For example, C<$!{ENOENT}> is true if and only if the current
1689 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1690 such file or directory" (or its moral equivalent: not all operating
1691 systems give that exact error, and certainly not all languages). To
1692 check if a particular key is meaningful on your system, use C<exists
1693 $!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1694 for more information, and also see L</$!>.
1696 This variable was added in Perl 5.005.
1701 X<$?> X<$CHILD_ERROR>
1703 The status returned by the last pipe close, backtick (C<``>) command,
1704 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1705 operator. This is just the 16-bit status word returned by the
1706 traditional Unix C<wait()> system call (or else is made up to look
1707 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1708 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1709 from, and C<$? & 128> reports whether there was a core dump.
1711 Additionally, if the C<h_errno> variable is supported in C, its value
1712 is returned via C<$?> if any C<gethost*()> function fails.
1714 If you have installed a signal handler for C<SIGCHLD>, the
1715 value of C<$?> will usually be wrong outside that handler.
1717 Inside an C<END> subroutine C<$?> contains the value that is going to be
1718 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1719 change the exit status of your program. For example:
1722 $? = 1 if $? == 255; # die would make it 255
1725 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1726 actual VMS exit status, instead of the default emulation of POSIX
1727 status; see L<perlvms/$?> for details.
1729 Mnemonic: similar to B<sh> and B<ksh>.
1734 X<$@> X<$EVAL_ERROR>
1736 The Perl syntax error message from the
1737 last C<eval()> operator. If C<$@> is
1738 the null string, the last C<eval()> parsed and executed correctly
1739 (although the operations you invoked may have failed in the normal
1742 Warning messages are not collected in this variable. You can, however,
1743 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1744 described in L</%SIG>.
1746 Mnemonic: Where was the syntax error "at"?
1750 =head2 Variables related to the interpreter state
1752 These variables provide information about the current interpreter state.
1759 X<$^C> X<$COMPILING>
1761 The current value of the flag associated with the B<-c> switch.
1762 Mainly of use with B<-MO=...> to allow code to alter its behavior
1763 when being compiled, such as for example to C<AUTOLOAD> at compile
1764 time rather than normal, deferred loading. Setting
1765 C<$^C = 1> is similar to calling C<B::minus_c>.
1767 This variable was added in Perl 5.6.
1772 X<$^D> X<$DEBUGGING>
1774 The current value of the debugging flags. May be read or set. Like its
1775 command-line equivalent, you can use numeric or symbolic values, eg
1776 C<$^D = 10> or C<$^D = "st">.
1778 Mnemonic: value of B<-D> switch.
1783 The I<object reference> to the C<Encode> object that is used to convert
1784 the source code to Unicode. Thanks to this variable your Perl script
1785 does not have to be written in UTF-8. Default is I<undef>. The direct
1786 manipulation of this variable is highly discouraged.
1788 This variable was added in Perl 5.8.2.
1790 =item ${^GLOBAL_PHASE}
1793 The current phase of the perl interpreter.
1795 Possible values are:
1801 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1802 value is mostly there for completeness and for use via the
1803 underlying C variable C<PL_phase>. It's not really possible for Perl
1804 code to be executed unless construction of the interpreter is
1809 This is the global compile-time. That includes, basically, every
1810 C<BEGIN> block executed directly or indirectly from during the
1811 compile-time of the top-level program.
1813 This phase is not called "BEGIN" to avoid confusion with
1814 C<BEGIN>-blocks, as those are executed during compile-time of any
1815 compilation unit, not just the top-level program. A new, localised
1816 compile-time entered at run-time, for example by constructs as
1817 C<eval "use SomeModule"> are not global interpreter phases, and
1818 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1822 Execution of any C<CHECK> blocks.
1826 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1830 The main run-time, i.e. the execution of C<PL_main_root>.
1834 Execution of any C<END> blocks.
1842 Also note that there's no value for UNITCHECK-blocks. That's because
1843 those are run for each compilation unit individually, and therefore is
1844 not a global interpreter phase.
1846 Not every program has to go through each of the possible phases, but
1847 transition from one phase to another can only happen in the order
1848 described in the above list.
1850 An example of all of the phases Perl code can see:
1852 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1854 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1856 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1859 package Print::Phase;
1862 my ($class, $time) = @_;
1863 return bless \$time, $class;
1868 print "$$self: ${^GLOBAL_PHASE}\n";
1872 print "run-time: ${^GLOBAL_PHASE}\n";
1874 my $runtime = Print::Phase->new(
1875 "lexical variables are garbage collected before END"
1878 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1880 our $destruct = Print::Phase->new(
1881 "package variables are garbage collected after END"
1890 lexical variables are garbage collected before END: RUN
1892 package variables are garbage collected after END: DESTRUCT
1894 This variable was added in Perl 5.14.0.
1899 WARNING: This variable is strictly for
1900 internal use only. Its availability,
1901 behavior, and contents are subject to change without notice.
1903 This variable contains compile-time hints for the Perl interpreter. At the
1904 end of compilation of a BLOCK the value of this variable is restored to the
1905 value when the interpreter started to compile the BLOCK.
1907 When perl begins to parse any block construct that provides a lexical scope
1908 (e.g., eval body, required file, subroutine body, loop body, or conditional
1909 block), the existing value of C<$^H> is saved, but its value is left unchanged.
1910 When the compilation of the block is completed, it regains the saved value.
1911 Between the points where its value is saved and restored, code that
1912 executes within BEGIN blocks is free to change the value of C<$^H>.
1914 This behavior provides the semantic of lexical scoping, and is used in,
1915 for instance, the C<use strict> pragma.
1917 The contents should be an integer; different bits of it are used for
1918 different pragmatic flags. Here's an example:
1920 sub add_100 { $^H |= 0x100 }
1927 Consider what happens during execution of the BEGIN block. At this point
1928 the BEGIN block has already been compiled, but the body of C<foo()> is still
1929 being compiled. The new value of C<$^H>
1930 will therefore be visible only while
1931 the body of C<foo()> is being compiled.
1933 Substitution of C<BEGIN { add_100() }> block with:
1935 BEGIN { require strict; strict->import('vars') }
1937 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1938 version of the same lexical pragma:
1941 require strict; strict->import('vars') if $condition
1944 This variable was added in Perl 5.003.
1949 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
1950 it useful for implementation of lexically scoped pragmas. See
1953 When putting items into C<%^H>, in order to avoid conflicting with other
1954 users of the hash there is a convention regarding which keys to use.
1955 A module should use only keys that begin with the module's name (the
1956 name of its main package) and a "/" character. For example, a module
1957 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
1959 This variable was added in Perl 5.6.
1964 An internal variable used by PerlIO. A string in two parts, separated
1965 by a C<\0> byte, the first part describes the input layers, the second
1966 part describes the output layers.
1968 This variable was added in Perl 5.8.0.
1975 The internal variable for debugging support. The meanings of the
1976 various bits are subject to change, but currently indicate:
1982 Debug subroutine enter/exit.
1986 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
1987 each statement executed. Also causes saving source code lines (like
1992 Switch off optimizations.
1996 Preserve more data for future interactive inspections.
2000 Keep info about source lines on which a subroutine is defined.
2004 Start with single-step on.
2008 Use subroutine address instead of name when reporting.
2012 Report C<goto &subroutine> as well.
2016 Provide informative "file" names for evals based on the place they were compiled.
2020 Provide informative names to anonymous subroutines based on the place they
2025 Save source code lines into C<@{"_<$filename"}>.
2029 Some bits may be relevant at compile-time only, some at
2030 run-time only. This is a new mechanism and the details may change.
2031 See also L<perldebguts>.
2036 Reflects if taint mode is on or off. 1 for on (the program was run with
2037 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2040 This variable is read-only.
2042 This variable was added in Perl 5.8.
2047 Reflects certain Unicode settings of Perl. See L<perlrun>
2048 documentation for the C<-C> switch for more information about
2049 the possible values.
2051 This variable is set during Perl startup and is thereafter read-only.
2053 This variable was added in Perl 5.8.2.
2058 This variable controls the state of the internal UTF-8 offset caching code.
2059 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2060 all its results against linear scans, and panicking on any discrepancy.
2062 This variable was added in Perl 5.8.9.
2064 =item ${^UTF8LOCALE}
2067 This variable indicates whether a UTF-8 locale was detected by perl at
2068 startup. This information is used by perl when it's in
2069 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2070 switch); see L<perlrun> for more info on this.
2072 This variable was added in Perl 5.8.8.
2076 =head2 Deprecated and removed variables
2078 Deprecating a variable announces the intent of the perl maintainers to
2079 eventually remove the variable from the language. It may still be
2080 available despite its status. Using a deprecated variable triggers
2083 Once a variable is removed, its use triggers an error telling you
2084 the variable is unsupported.
2086 See L<perldiag> for details about error messages.
2095 C<$#> was a variable that could be used to format printed numbers.
2096 After a deprecation cycle, its magic was removed in Perl 5.10 and
2097 using it now triggers a warning: C<$# is no longer supported>.
2099 This is not the sigil you use in front of an array name to get the
2100 last index, like C<$#array>. That's still how you get the last index
2101 of an array in Perl. The two have nothing to do with each other.
2103 Deprecated in Perl 5.
2105 Removed in Perl 5.10.
2110 C<$*> was a variable that you could use to enable multiline matching.
2111 After a deprecation cycle, its magic was removed in Perl 5.10.
2112 Using it now triggers a warning: C<$* is no longer supported>.
2113 You should use the C</s> and C</m> regexp modifiers instead.
2115 Deprecated in Perl 5.
2117 Removed in Perl 5.10.
2122 X<$[> X<$ARRAY_BASE>
2124 This variable stores the index of the first element in an array, and
2125 of the first character in a substring. The default is 0, but you could
2126 theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2127 when subscripting and when evaluating the index() and substr() functions.
2129 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2130 directive, and cannot influence the behavior of any other file.
2131 (That's why you can only assign compile-time constants to it.)
2132 Its use is highly discouraged.
2134 Prior to Perl 5.10, assignment to C<$[> could be seen from outer lexical
2135 scopes in the same file, unlike other compile-time directives (such as
2136 L<strict>). Using local() on it would bind its value strictly to a lexical
2137 block. Now it is always lexically scoped.
2139 As of Perl 5.16, it is implemented by the L<arybase> module. See
2140 L<arybase> for more details on its behaviour.
2142 Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2143 effect, and always contains 0. Assigning 0 to it is permitted, but any
2144 other value will produce an error.
2146 Mnemonic: [ begins subscripts.
2148 Deprecated in Perl 5.12.
2150 =item $OLD_PERL_VERSION
2153 X<$]> X<$OLD_PERL_VERSION>
2155 See L</$^V> for a more modern representation of the Perl version that allows
2156 accurate string comparisons.
2158 The version + patchlevel / 1000 of the Perl interpreter. This variable
2159 can be used to determine whether the Perl interpreter executing a
2160 script is in the right range of versions:
2162 warn "No checksumming!\n" if $] < 3.019;
2164 The floating point representation can sometimes lead to inaccurate
2165 numeric comparisons.
2167 See also the documentation of C<use VERSION> and C<require VERSION>
2168 for a convenient way to fail if the running Perl interpreter is too old.
2170 Mnemonic: Is this version of perl in the right bracket?