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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
b0c22438 7=head2 The Syntax of Variable Names
8
241a59d9 9Variable names in Perl can have several formats. Usually, they
b0c22438 10must begin with a letter or underscore, in which case they can be
11arbitrarily long (up to an internal limit of 251 characters) and
12may contain letters, digits, underscores, or the special sequence
241a59d9 13C<::> or C<'>. In this case, the part before the last C<::> or
b0c22438 14C<'> is taken to be a I<package qualifier>; see L<perlmod>.
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15A Unicode letter that is not ASCII is not considered to be a letter
16unless S<C<"use utf8">> is in effect, and somewhat more complicated
17rules apply; see L<perldata/Identifier parsing> for details.
b0c22438 18
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19Perl variable names may also be a sequence of digits, a single
20punctuation character, or the two-character sequence: C<^> (caret or
21CIRCUMFLEX ACCENT) followed by any one of the characters C<[][A-Z^_?\]>.
22These names are all reserved for
b0c22438 23special uses by Perl; for example, the all-digits names are used
24to hold data captured by backreferences after a regular expression
ce4793f1 25match.
b0c22438 26
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27Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
28preceded by a caret. These must all be written in the form C<${^Foo}>;
29the braces are not optional. C<${^Foo}> denotes the scalar variable
30whose name is considered to be a control-C<F> followed by two C<o>'s.
31These variables are
b0c22438 32reserved for future special uses by Perl, except for the ones that
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33begin with C<^_> (caret-underscore). No
34name that begins with C<^_> will acquire a special
b0c22438 35meaning in any future version of Perl; such names may therefore be
241a59d9 36used safely in programs. C<$^_> itself, however, I<is> reserved.
b0c22438 37
ce4793f1 38Perl identifiers that begin with digits or
b0c22438 39punctuation characters are exempt from the effects of the C<package>
40declaration and are always forced to be in package C<main>; they are
241a59d9 41also exempt from C<strict 'vars'> errors. A few other names are also
b0c22438 42exempt in these ways:
43
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44 ENV STDIN
45 INC STDOUT
46 ARGV STDERR
47 ARGVOUT
48 SIG
b0c22438 49
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
53
54=head1 SPECIAL VARIABLES
a0d0e21e 55
241a59d9 56The following names have special meaning to Perl. Most punctuation
0b9346e6 57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
9548c15c 60 use English;
a0d0e21e 61
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62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
1e7d0944 64borrowed from B<awk>. For more info, please see L<English>.
a1ce9542 65
241a59d9 66Before you continue, note the sort order for variables. In general, we
0b9346e6 67first list the variables in case-insensitive, almost-lexigraphical
68order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
69or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
70For variables with the same identifier, we list it in order of scalar,
71array, hash, and bareword.
a1ce9542 72
b0c22438 73=head2 General Variables
a0d0e21e 74
84dabc03 75=over 8
76
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77=item $ARG
78
79=item $_
a054c801 80X<$_> X<$ARG>
a0d0e21e 81
241a59d9 82The default input and pattern-searching space. The following pairs are
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83equivalent:
84
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85 while (<>) {...} # equivalent only in while!
86 while (defined($_ = <>)) {...}
a0d0e21e 87
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88 /^Subject:/
89 $_ =~ /^Subject:/
a0d0e21e 90
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91 tr/a-z/A-Z/
92 $_ =~ tr/a-z/A-Z/
a0d0e21e 93
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94 chomp
95 chomp($_)
a0d0e21e 96
0b9346e6 97Here are the places where Perl will assume C<$_> even if you don't use it:
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98
99=over 3
100
101=item *
102
84dabc03 103The following functions use C<$_> as a default argument:
db1511c8 104
f61f53cc 105abs, alarm, chomp, chop, chr, chroot,
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106cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
107lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
b0169937 108quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
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109rmdir, say, sin, split (for its second
110argument), sqrt, stat, study, uc, ucfirst,
b0169937 111unlink, unpack.
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112
113=item *
114
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115All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
116See L<perlfunc/-X>
117
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118=item *
119
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120The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
121when used without an C<=~> operator.
cb1a09d0 122
54310121 123=item *
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124
125The default iterator variable in a C<foreach> loop if no other
126variable is supplied.
127
54310121 128=item *
cb1a09d0 129
b0c22438 130The implicit iterator variable in the C<grep()> and C<map()> functions.
cb1a09d0 131
54310121 132=item *
cb1a09d0 133
b0c22438 134The implicit variable of C<given()>.
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135
136=item *
137
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138The default place to put the next value or input record
139when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
cb1a09d0 140operation's result is tested by itself as the sole criterion of a C<while>
241a59d9 141test. Outside a C<while> test, this will not happen.
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142
143=back
144
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145C<$_> is by default a global variable. However, as
146of perl v5.10.0, you can use a lexical version of
241a59d9 147C<$_> by declaring it in a file or in a block with C<my>. Moreover,
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148declaring C<our $_> restores the global C<$_> in the current scope. Though
149this seemed like a good idea at the time it was introduced, lexical C<$_>
150actually causes more problems than it solves. If you call a function that
151expects to be passed information via C<$_>, it may or may not work,
152depending on how the function is written, there not being any easy way to
153solve this. Just avoid lexical C<$_>, unless you are feeling particularly
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154masochistic. For this reason lexical C<$_> is still experimental and will
155produce a warning unless warnings have been disabled. As with other
156experimental features, the behavior of lexical C<$_> is subject to change
157without notice, including change into a fatal error.
59f00321 158
b0c22438 159Mnemonic: underline is understood in certain operations.
a0d0e21e 160
0b9346e6 161=item @ARG
cde0cee5 162
0b9346e6 163=item @_
164X<@_> X<@ARG>
a0d0e21e 165
0b9346e6 166Within a subroutine the array C<@_> contains the parameters passed to
241a59d9 167that subroutine. Inside a subroutine, C<@_> is the default array for
256ca3d3 168the array operators C<pop> and C<shift>.
a0d0e21e 169
0b9346e6 170See L<perlsub>.
a0d0e21e 171
1311257d 172=item $LIST_SEPARATOR
173
174=item $"
175X<$"> X<$LIST_SEPARATOR>
176
69520822 177When an array or an array slice is interpolated into a double-quoted
178string or a similar context such as C</.../>, its elements are
241a59d9 179separated by this value. Default is a space. For example, this:
69520822 180
9548c15c 181 print "The array is: @array\n";
69520822 182
183is equivalent to this:
184
9548c15c 185 print "The array is: " . join($", @array) . "\n";
69520822 186
187Mnemonic: works in double-quoted context.
1311257d 188
b0c22438 189=item $PROCESS_ID
cde0cee5 190
b0c22438 191=item $PID
a0d0e21e 192
b0c22438 193=item $$
194X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 195
241a59d9 196The process number of the Perl running this script. Though you I<can> set
4a904372 197this variable, doing so is generally discouraged, although it can be
241a59d9 198invaluable for some testing purposes. It will be reset automatically
b0c22438 199across C<fork()> calls.
a0d0e21e 200
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201Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
202would emulate POSIX semantics on Linux systems using LinuxThreads, a
203partial implementation of POSIX Threads that has since been superseded
204by the Native POSIX Thread Library (NPTL).
205
e3f68f70 206LinuxThreads is now obsolete on Linux, and caching C<getpid()>
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207like this made embedding perl unnecessarily complex (since you'd have
208to manually update the value of $$), so now C<$$> and C<getppid()>
209will always return the same values as the underlying C library.
210
211Debian GNU/kFreeBSD systems also used LinuxThreads up until and
212including the 6.0 release, but after that moved to FreeBSD thread
213semantics, which are POSIX-like.
214
215To see if your system is affected by this discrepancy check if
216C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
1e7d0944 217value. NTPL threads preserve the POSIX semantics.
a0d0e21e 218
b0c22438 219Mnemonic: same as shells.
ad83b128 220
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221=item $PROGRAM_NAME
222
223=item $0
224X<$0> X<$PROGRAM_NAME>
225
226Contains the name of the program being executed.
227
228On some (but not all) operating systems assigning to C<$0> modifies
241a59d9 229the argument area that the C<ps> program sees. On some platforms you
66d7055b 230may have to use special C<ps> options or a different C<ps> to see the
241a59d9 231changes. Modifying the C<$0> is more useful as a way of indicating the
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232current program state than it is for hiding the program you're
233running.
234
235Note that there are platform-specific limitations on the maximum
241a59d9 236length of C<$0>. In the most extreme case it may be limited to the
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237space occupied by the original C<$0>.
238
239In some platforms there may be arbitrary amount of padding, for
240example space characters, after the modified name as shown by C<ps>.
241In some platforms this padding may extend all the way to the original
242length of the argument area, no matter what you do (this is the case
243for example with Linux 2.2).
244
245Note for BSD users: setting C<$0> does not completely remove "perl"
241a59d9 246from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
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247result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
248and the " (perl)" suffix are shown depends on your exact BSD variant
241a59d9 249and version). This is an operating system feature, Perl cannot help it.
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250
251In multithreaded scripts Perl coordinates the threads so that any
252thread may modify its copy of the C<$0> and the change becomes visible
241a59d9 253to ps(1) (assuming the operating system plays along). Note that
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254the view of C<$0> the other threads have will not change since they
255have their own copies of it.
256
257If the program has been given to perl via the switches C<-e> or C<-E>,
258C<$0> will contain the string C<"-e">.
259
60cf4914 260On Linux as of perl v5.14.0 the legacy process name will be set with
66d7055b 261C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
241a59d9 262perl has done since version 4.000. Now system utilities that read the
66d7055b 263legacy process name such as ps, top and killall will recognize the
241a59d9 264name you set when assigning to C<$0>. The string you supply will be
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265cut off at 16 bytes, this is a limitation imposed by Linux.
266
267Mnemonic: same as B<sh> and B<ksh>.
268
b0c22438 269=item $REAL_GROUP_ID
a01268b5 270
b0c22438 271=item $GID
a01268b5 272
b0c22438 273=item $(
274X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 275
241a59d9 276The real gid of this process. If you are on a machine that supports
b0c22438 277membership in multiple groups simultaneously, gives a space separated
241a59d9 278list of groups you are in. The first number is the one returned by
b0c22438 279C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
280the same as the first number.
a01268b5 281
b0c22438 282However, a value assigned to C<$(> must be a single number used to
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283set the real gid. So the value given by C<$(> should I<not> be assigned
284back to C<$(> without being forced numeric, such as by adding zero. Note
b0c22438 285that this is different to the effective gid (C<$)>) which does take a
286list.
fe307981 287
b0c22438 288You can change both the real gid and the effective gid at the same
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289time by using C<POSIX::setgid()>. Changes
290to C<$(> require a check to C<$!>
b0c22438 291to detect any possible errors after an attempted change.
6cef1e77 292
241a59d9 293Mnemonic: parentheses are used to I<group> things. The real gid is the
b0c22438 294group you I<left>, if you're running setgid.
6cef1e77 295
b0c22438 296=item $EFFECTIVE_GROUP_ID
8e08999f 297
b0c22438 298=item $EGID
81714fb9 299
b0c22438 300=item $)
301X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
81714fb9 302
241a59d9 303The effective gid of this process. If you are on a machine that
b0c22438 304supports membership in multiple groups simultaneously, gives a space
241a59d9 305separated list of groups you are in. The first number is the one
b0c22438 306returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
307one of which may be the same as the first number.
81714fb9 308
b0c22438 309Similarly, a value assigned to C<$)> must also be a space-separated
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310list of numbers. The first number sets the effective gid, and
311the rest (if any) are passed to C<setgroups()>. To get the effect of an
b0c22438 312empty list for C<setgroups()>, just repeat the new effective gid; that is,
313to force an effective gid of 5 and an effectively empty C<setgroups()>
314list, say C< $) = "5 5" >.
81714fb9 315
b0c22438 316You can change both the effective gid and the real gid at the same
317time by using C<POSIX::setgid()> (use only a single numeric argument).
318Changes to C<$)> require a check to C<$!> to detect any possible errors
319after an attempted change.
44a2ac75 320
b0c22438 321C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
241a59d9 322machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
b0c22438 323and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 324
241a59d9 325Mnemonic: parentheses are used to I<group> things. The effective gid
b0c22438 326is the group that's I<right> for you, if you're running setgid.
44a2ac75 327
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328=item $REAL_USER_ID
329
330=item $UID
331
332=item $<
333X<< $< >> X<$UID> X<$REAL_USER_ID>
334
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335The real uid of this process. You can change both the real uid and the
336effective uid at the same time by using C<POSIX::setuid()>. Since
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337changes to C<< $< >> require a system call, check C<$!> after a change
338attempt to detect any possible errors.
339
340Mnemonic: it's the uid you came I<from>, if you're running setuid.
341
342=item $EFFECTIVE_USER_ID
343
344=item $EUID
345
346=item $>
347X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
348
241a59d9 349The effective uid of this process. For example:
c82f2f4e 350
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351 $< = $>; # set real to effective uid
352 ($<,$>) = ($>,$<); # swap real and effective uids
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353
354You can change both the effective uid and the real uid at the same
241a59d9 355time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
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356to C<$!> to detect any possible errors after an attempted change.
357
358C<< $< >> and C<< $> >> can be swapped only on machines
359supporting C<setreuid()>.
360
361Mnemonic: it's the uid you went I<to>, if you're running setuid.
362
0b9346e6 363=item $SUBSCRIPT_SEPARATOR
364
365=item $SUBSEP
366
367=item $;
368X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
369
241a59d9 370The subscript separator for multidimensional array emulation. If you
0b9346e6 371refer to a hash element as
372
592708b4 373 $foo{$x,$y,$z}
0b9346e6 374
375it really means
376
592708b4 377 $foo{join($;, $x, $y, $z)}
0b9346e6 378
379But don't put
380
592708b4 381 @foo{$x,$y,$z} # a slice--note the @
0b9346e6 382
383which means
384
592708b4 385 ($foo{$x},$foo{$y},$foo{$z})
0b9346e6 386
241a59d9 387Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 388binary data there might not be any safe value for C<$;>.
389
390Consider using "real" multidimensional arrays as described
391in L<perllol>.
392
393Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
394
0b9346e6 395=item $a
396
397=item $b
398X<$a> X<$b>
399
400Special package variables when using C<sort()>, see L<perlfunc/sort>.
401Because of this specialness C<$a> and C<$b> don't need to be declared
402(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
241a59d9 403pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
0b9346e6 404be able to use them in the C<sort()> comparison block or function.
405
0b9346e6 406=item %ENV
407X<%ENV>
408
241a59d9 409The hash C<%ENV> contains your current environment. Setting a
0b9346e6 410value in C<ENV> changes the environment for any child processes
411you subsequently C<fork()> off.
412
32e006ac 413As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
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414
415 my $foo = 1;
416 $ENV{'bar'} = \$foo;
417 if( ref $ENV{'bar'} ) {
32e006ac 418 say "Pre 5.18.0 Behaviour";
a5effbbc 419 } else {
32e006ac 420 say "Post 5.18.0 Behaviour";
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421 }
422
423Previously, only child processes received stringified values:
424
425 my $foo = 1;
426 $ENV{'bar'} = \$foo;
427
428 # Always printed 'non ref'
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429 system($^X, '-e',
430 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
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431
432This happens because you can't really share arbitrary data structures with
433foreign processes.
434
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435=item $OLD_PERL_VERSION
436
4ad0ecd4 437=item $]
b77ebf74 438X<$]> X<$OLD_PERL_VERSION>
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439
440The revision, version, and subversion of the Perl interpreter, represented
441as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
442is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
443
444This variable can be used to determine whether the Perl interpreter
445executing a script is in the right range of versions:
446
447 warn "No PerlIO!\n" if $] lt '5.008';
448
449When comparing C<$]>, string comparison operators are B<highly
450recommended>. The inherent limitations of binary floating point
451representation can sometimes lead to incorrect comparisons for some
452numbers on some architectures.
453
454See also the documentation of C<use VERSION> and C<require VERSION>
455for a convenient way to fail if the running Perl interpreter is too old.
456
457See L</$^V> for a representation of the Perl version as a L<version>
458object, which allows more flexible string comparisons.
459
460The main advantage of C<$]> over C<$^V> is that it works the same on any
461version of Perl. The disadvantages are that it can't easily be compared
462to versions in other formats (e.g. literal v-strings, "v1.2.3" or
463version objects) and numeric comparisons can occasionally fail; it's good
464for string literal version checks and bad for comparing to a variable
465that hasn't been sanity-checked.
466
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467The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
468reasons but its use is discouraged. (If your reason to use C<$]> is to
469run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
470be self-defeating.)
4ad0ecd4 471
9fcc2314 472Mnemonic: Is this version of perl in the right bracket?
83c1fffe 473
b0c22438 474=item $SYSTEM_FD_MAX
5b2b9c68 475
b0c22438 476=item $^F
477X<$^F> X<$SYSTEM_FD_MAX>
5b2b9c68 478
241a59d9 479The maximum system file descriptor, ordinarily 2. System file
b0c22438 480descriptors are passed to C<exec()>ed processes, while higher file
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481descriptors are not. Also, during an
482C<open()>, system file descriptors are
b0c22438 483preserved even if the C<open()> fails (ordinary file descriptors are
241a59d9 484closed before the C<open()> is attempted). The close-on-exec
b0c22438 485status of a file descriptor will be decided according to the value of
486C<$^F> when the corresponding file, pipe, or socket was opened, not the
487time of the C<exec()>.
5b2b9c68 488
0b9346e6 489=item @F
490X<@F>
491
492The array C<@F> contains the fields of each line read in when autosplit
241a59d9 493mode is turned on. See L<perlrun> for the B<-a> switch. This array
0b9346e6 494is package-specific, and must be declared or given a full package name
495if not in package main when running under C<strict 'vars'>.
496
0b9346e6 497=item @INC
498X<@INC>
499
500The array C<@INC> contains the list of places that the C<do EXPR>,
241a59d9 501C<require>, or C<use> constructs look for their library files. It
0b9346e6 502initially consists of the arguments to any B<-I> command-line
503switches, followed by the default Perl library, probably
504F</usr/local/lib/perl>, followed by ".", to represent the current
241a59d9 505directory. ("." will not be appended if taint checks are enabled,
0b9346e6 506either by C<-T> or by C<-t>.) If you need to modify this at runtime,
507you should use the C<use lib> pragma to get the machine-dependent
508library properly loaded also:
509
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510 use lib '/mypath/libdir/';
511 use SomeMod;
0b9346e6 512
513You can also insert hooks into the file inclusion system by putting Perl
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514code directly into C<@INC>. Those hooks may be subroutine references,
515array references or blessed objects. See L<perlfunc/require> for details.
0b9346e6 516
517=item %INC
518X<%INC>
519
520The hash C<%INC> contains entries for each filename included via the
241a59d9 521C<do>, C<require>, or C<use> operators. The key is the filename
0b9346e6 522you specified (with module names converted to pathnames), and the
241a59d9 523value is the location of the file found. The C<require>
0b9346e6 524operator uses this hash to determine whether a particular file has
525already been included.
526
527If the file was loaded via a hook (e.g. a subroutine reference, see
528L<perlfunc/require> for a description of these hooks), this hook is
241a59d9 529by default inserted into C<%INC> in place of a filename. Note, however,
0b9346e6 530that the hook may have set the C<%INC> entry by itself to provide some more
531specific info.
532
b0c22438 533=item $INPLACE_EDIT
a0d0e21e 534
b0c22438 535=item $^I
536X<$^I> X<$INPLACE_EDIT>
a0d0e21e 537
241a59d9 538The current value of the inplace-edit extension. Use C<undef> to disable
b0c22438 539inplace editing.
a0d0e21e 540
b0c22438 541Mnemonic: value of B<-i> switch.
a0d0e21e 542
b0c22438 543=item $^M
544X<$^M>
a0d0e21e 545
b0c22438 546By default, running out of memory is an untrappable, fatal error.
547However, if suitably built, Perl can use the contents of C<$^M>
241a59d9 548as an emergency memory pool after C<die()>ing. Suppose that your Perl
b0c22438 549were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
550Then
a0d0e21e 551
9548c15c 552 $^M = 'a' x (1 << 16);
a0d0e21e 553
241a59d9 554would allocate a 64K buffer for use in an emergency. See the
b0c22438 555F<INSTALL> file in the Perl distribution for information on how to
241a59d9 556add custom C compilation flags when compiling perl. To discourage casual
b0c22438 557use of this advanced feature, there is no L<English|English> long name for
558this variable.
a0d0e21e 559
b0c22438 560This variable was added in Perl 5.004.
a0d0e21e 561
b0c22438 562=item $OSNAME
a0d0e21e 563
b0c22438 564=item $^O
565X<$^O> X<$OSNAME>
a0d0e21e 566
b0c22438 567The name of the operating system under which this copy of Perl was
241a59d9 568built, as determined during the configuration process. For examples
b0c22438 569see L<perlport/PLATFORMS>.
a0d0e21e 570
241a59d9 571The value is identical to C<$Config{'osname'}>. See also L<Config>
b0c22438 572and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 573
b0c22438 574In Windows platforms, C<$^O> is not very helpful: since it is always
575C<MSWin32>, it doesn't tell the difference between
241a59d9 57695/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
b0c22438 577Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
578between the variants.
a0d0e21e 579
b0c22438 580This variable was added in Perl 5.003.
a0d0e21e 581
1fa81471
DR
582=item %SIG
583X<%SIG>
a0d0e21e 584
241a59d9 585The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 586
9548c15c
FC
587 sub handler { # 1st argument is signal name
588 my($sig) = @_;
589 print "Caught a SIG$sig--shutting down\n";
590 close(LOG);
591 exit(0);
592 }
a0d0e21e 593
9548c15c
FC
594 $SIG{'INT'} = \&handler;
595 $SIG{'QUIT'} = \&handler;
596 ...
597 $SIG{'INT'} = 'DEFAULT'; # restore default action
598 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 599
1fa81471 600Using a value of C<'IGNORE'> usually has the effect of ignoring the
241a59d9 601signal, except for the C<CHLD> signal. See L<perlipc> for more about
1fa81471 602this special case.
a0d0e21e 603
1fa81471 604Here are some other examples:
a0d0e21e 605
9548c15c
FC
606 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
607 # recommended)
608 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
609 # Plumber
610 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
611 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
612 # return??
a0d0e21e 613
1fa81471
DR
614Be sure not to use a bareword as the name of a signal handler,
615lest you inadvertently call it.
a0d0e21e 616
1fa81471 617If your system has the C<sigaction()> function then signal handlers
241a59d9 618are installed using it. This means you get reliable signal handling.
a0d0e21e 619
60cf4914 620The default delivery policy of signals changed in Perl v5.8.0 from
1fa81471 621immediate (also known as "unsafe") to deferred, also known as "safe
241a59d9 622signals". See L<perlipc> for more information.
a0d0e21e 623
241a59d9 624Certain internal hooks can be also set using the C<%SIG> hash. The
1fa81471 625routine indicated by C<$SIG{__WARN__}> is called when a warning
241a59d9
FC
626message is about to be printed. The warning message is passed as the
627first argument. The presence of a C<__WARN__> hook causes the
628ordinary printing of warnings to C<STDERR> to be suppressed. You can
1fa81471
DR
629use this to save warnings in a variable, or turn warnings into fatal
630errors, like this:
a0d0e21e 631
9548c15c
FC
632 local $SIG{__WARN__} = sub { die $_[0] };
633 eval $proggie;
a8f8344d 634
b0c22438 635As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
636disable warnings using the empty subroutine:
f86702cc 637
9548c15c 638 local $SIG{__WARN__} = sub {};
55602bd2 639
b0c22438 640The routine indicated by C<$SIG{__DIE__}> is called when a fatal
241a59d9
FC
641exception is about to be thrown. The error message is passed as the
642first argument. When a C<__DIE__> hook routine returns, the exception
b0c22438 643processing continues as it would have in the absence of the hook,
c94b42ea
DM
644unless the hook routine itself exits via a C<goto &sub>, a loop exit,
645or a C<die()>. The C<__DIE__> handler is explicitly disabled during
646the call, so that you can die from a C<__DIE__> handler. Similarly
647for C<__WARN__>.
e5218da5 648
b0c22438 649Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
241a59d9 650even inside an C<eval()>. Do not use this to rewrite a pending
b0c22438 651exception in C<$@>, or as a bizarre substitute for overriding
241a59d9 652C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
b0c22438 653in a future release so that C<$SIG{__DIE__}> is only called if your
241a59d9 654program is about to exit, as was the original intent. Any other use is
b0c22438 655deprecated.
656
657C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
241a59d9 658may be called to report (probable) errors found by the parser. In such
b0c22438 659a case the parser may be in inconsistent state, so any attempt to
660evaluate Perl code from such a handler will probably result in a
241a59d9 661segfault. This means that warnings or errors that result from parsing
b0c22438 662Perl should be used with extreme caution, like this:
e5218da5 663
9548c15c
FC
664 require Carp if defined $^S;
665 Carp::confess("Something wrong") if defined &Carp::confess;
666 die "Something wrong, but could not load Carp to give "
667 . "backtrace...\n\t"
668 . "To see backtrace try starting Perl with -MCarp switch";
e5218da5 669
b0c22438 670Here the first line will load C<Carp> I<unless> it is the parser who
241a59d9
FC
671called the handler. The second line will print backtrace and die if
672C<Carp> was available. The third line will be executed only if C<Carp> was
b0c22438 673not available.
0a378802 674
0b9346e6 675Having to even think about the C<$^S> variable in your exception
241a59d9
FC
676handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
677invites grievous and difficult to track down errors. Avoid it
0b9346e6 678and use an C<END{}> or CORE::GLOBAL::die override instead.
679
b0c22438 680See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
681L<warnings> for additional information.
0a378802 682
b0c22438 683=item $BASETIME
6ab308ee 684
b0c22438 685=item $^T
686X<$^T> X<$BASETIME>
6ab308ee 687
b0c22438 688The time at which the program began running, in seconds since the
241a59d9 689epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
b0c22438 690and B<-C> filetests are based on this value.
a0d0e21e 691
b0c22438 692=item $PERL_VERSION
a0d0e21e 693
b0c22438 694=item $^V
695X<$^V> X<$PERL_VERSION>
a0d0e21e 696
b0c22438 697The revision, version, and subversion of the Perl interpreter,
eb82332c 698represented as a L<version> object.
748a9306 699
60cf4914
BF
700This variable first appeared in perl v5.6.0; earlier versions of perl
701will see an undefined value. Before perl v5.10.0 C<$^V> was represented
f20d3573 702as a v-string rather than a L<version> object.
55602bd2 703
b0c22438 704C<$^V> can be used to determine whether the Perl interpreter executing
241a59d9 705a script is in the right range of versions. For example:
a0d0e21e 706
9548c15c 707 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 708
f20d3573
DG
709While version objects overload stringification, to portably convert
710C<$^V> into its string representation, use C<sprintf()>'s C<"%vd">
711conversion, which works for both v-strings or version objects:
a0d0e21e 712
9548c15c 713 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 714
b0c22438 715See the documentation of C<use VERSION> and C<require VERSION>
716for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 717
6a29646e 718See also C<L</$]>> for a decimal representation of the Perl version.
a0d0e21e 719
f20d3573
DG
720The main advantage of C<$^V> over C<$]> is that, for Perl v5.10.0 or
721later, it overloads operators, allowing easy comparison against other
722version representations (e.g. decimal, literal v-string, "v1.2.3", or
723objects). The disadvantage is that prior to v5.10.0, it was only a
6a29646e
KW
724literal v-string, which can't be easily printed or compared, whereas
725the behavior of C<$]> is unchanged on all versions of Perl.
a0d0e21e 726
eb82332c 727Mnemonic: use ^V for a version object.
a0d0e21e 728
b0c22438 729=item ${^WIN32_SLOPPY_STAT}
5b442a2a 730X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
a0d0e21e 731
b0c22438 732If this variable is set to a true value, then C<stat()> on Windows will
241a59d9 733not try to open the file. This means that the link count cannot be
b0c22438 734determined and file attributes may be out of date if additional
241a59d9 735hardlinks to the file exist. On the other hand, not opening the file
b0c22438 736is considerably faster, especially for files on network drives.
a0d0e21e 737
b0c22438 738This variable could be set in the F<sitecustomize.pl> file to
739configure the local Perl installation to use "sloppy" C<stat()> by
241a59d9 740default. See the documentation for B<-f> in
b0c22438 741L<perlrun|perlrun/"Command Switches"> for more information about site
742customization.
a0d0e21e 743
60cf4914 744This variable was added in Perl v5.10.0.
a0d0e21e 745
b0c22438 746=item $EXECUTABLE_NAME
a0d0e21e 747
b0c22438 748=item $^X
749X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 750
b0c22438 751The name used to execute the current copy of Perl, from C's
752C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 753
b0c22438 754Depending on the host operating system, the value of C<$^X> may be
755a relative or absolute pathname of the perl program file, or may
756be the string used to invoke perl but not the pathname of the
241a59d9 757perl program file. Also, most operating systems permit invoking
b0c22438 758programs that are not in the PATH environment variable, so there
241a59d9 759is no guarantee that the value of C<$^X> is in PATH. For VMS, the
b0c22438 760value may or may not include a version number.
a0d0e21e 761
b0c22438 762You usually can use the value of C<$^X> to re-invoke an independent
763copy of the same perl that is currently running, e.g.,
a0d0e21e 764
9548c15c 765 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 766
b0c22438 767But recall that not all operating systems support forking or
768capturing of the output of commands, so this complex statement
769may not be portable.
a0d0e21e 770
b0c22438 771It is not safe to use the value of C<$^X> as a path name of a file,
772as some operating systems that have a mandatory suffix on
773executable files do not require use of the suffix when invoking
241a59d9 774a command. To convert the value of C<$^X> to a path name, use the
b0c22438 775following statements:
8cc95fdb 776
9548c15c
FC
777 # Build up a set of file names (not command names).
778 use Config;
779 my $this_perl = $^X;
780 if ($^O ne 'VMS') {
781 $this_perl .= $Config{_exe}
782 unless $this_perl =~ m/$Config{_exe}$/i;
783 }
8cc95fdb 784
b0c22438 785Because many operating systems permit anyone with read access to
786the Perl program file to make a copy of it, patch the copy, and
787then execute the copy, the security-conscious Perl programmer
788should take care to invoke the installed copy of perl, not the
241a59d9 789copy referenced by C<$^X>. The following statements accomplish
b0c22438 790this goal, and produce a pathname that can be invoked as a
791command or referenced as a file.
a043a685 792
9548c15c
FC
793 use Config;
794 my $secure_perl_path = $Config{perlpath};
795 if ($^O ne 'VMS') {
796 $secure_perl_path .= $Config{_exe}
797 unless $secure_perl_path =~ m/$Config{_exe}$/i;
798 }
a0d0e21e 799
b0c22438 800=back
a0d0e21e 801
b0c22438 802=head2 Variables related to regular expressions
803
804Most of the special variables related to regular expressions are side
241a59d9
FC
805effects. Perl sets these variables when it has a successful match, so
806you should check the match result before using them. For instance:
b0c22438 807
9548c15c
FC
808 if( /P(A)TT(ER)N/ ) {
809 print "I found $1 and $2\n";
810 }
b0c22438 811
0b9346e6 812These variables are read-only and dynamically-scoped, unless we note
b0c22438 813otherwise.
814
0b9346e6 815The dynamic nature of the regular expression variables means that
816their value is limited to the block that they are in, as demonstrated
817by this bit of code:
b0c22438 818
9548c15c
FC
819 my $outer = 'Wallace and Grommit';
820 my $inner = 'Mutt and Jeff';
0b9346e6 821
9548c15c 822 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 823
9548c15c 824 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 825
9548c15c
FC
826 {
827 OUTER:
828 show_n() if $outer =~ m/$pattern/;
0b9346e6 829
9548c15c
FC
830 INNER: {
831 show_n() if $inner =~ m/$pattern/;
832 }
0b9346e6 833
9548c15c
FC
834 show_n();
835 }
b0c22438 836
0b9346e6 837The output shows that while in the C<OUTER> block, the values of C<$1>
241a59d9 838and C<$2> are from the match against C<$outer>. Inside the C<INNER>
0b9346e6 839block, the values of C<$1> and C<$2> are from the match against
840C<$inner>, but only until the end of the block (i.e. the dynamic
241a59d9 841scope). After the C<INNER> block completes, the values of C<$1> and
0b9346e6 842C<$2> return to the values for the match against C<$outer> even though
b0c22438 843we have not made another match:
844
9548c15c
FC
845 $1 is Wallace; $2 is Grommit
846 $1 is Mutt; $2 is Jeff
847 $1 is Wallace; $2 is Grommit
a0d0e21e 848
40445027 849=head3 Performance issues
0b9346e6 850
40445027
DM
851Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
852or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
853all subsequent successful pattern matches to make a copy of the matched
854string, in case the code might subsequently access one of those variables.
855This imposed a considerable performance penalty across the whole program,
856so generally the use of these variables has been discouraged.
0b9346e6 857
40445027
DM
858In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
859supply the indices of successful matches. So you could for example do
860this:
861
862 $str =~ /pattern/;
863
864 print $`, $&, $'; # bad: perfomance hit
865
866 print # good: no perfomance hit
867 substr($str, 0, $-[0]),
868 substr($str, $-[0], $+[0]-$-[0]),
869 substr($str, $+[0]);
870
871In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
872C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
873you to suffer the penalties only on patterns marked with C</p>.
874
875In Perl 5.18.0 onwards, perl started noting the presence of each of the
876three variables separately, and only copied that part of the string
877required; so in
878
879 $`; $&; "abcdefgh" =~ /d/
880
881perl would only copy the "abcd" part of the string. That could make a big
882difference in something like
883
884 $str = 'x' x 1_000_000;
885 $&; # whoops
886 $str =~ /x/g # one char copied a million times, not a million chars
887
888In Perl 5.20.0 a new copy-on-write system was enabled by default, which
889finally fixes all performance issues with these three variables, and makes
890them safe to use anywhere.
891
892The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
893find uses of these problematic match variables in your code.
13b0f67d 894
b0c22438 895=over 8
a0d0e21e 896
b0c22438 897=item $<I<digits>> ($1, $2, ...)
898X<$1> X<$2> X<$3>
8cc95fdb 899
b0c22438 900Contains the subpattern from the corresponding set of capturing
901parentheses from the last successful pattern match, not counting patterns
902matched in nested blocks that have been exited already.
8cc95fdb 903
b0c22438 904These variables are read-only and dynamically-scoped.
a043a685 905
b0c22438 906Mnemonic: like \digits.
a0d0e21e 907
b0c22438 908=item $MATCH
a0d0e21e 909
b0c22438 910=item $&
911X<$&> X<$MATCH>
a0d0e21e 912
b0c22438 913The string matched by the last successful pattern match (not counting
914any matches hidden within a BLOCK or C<eval()> enclosed by the current
915BLOCK).
a0d0e21e 916
40445027
DM
917See L</Performance issues> above for the serious performance implications
918of using this variable (even once) in your code.
80bca1b4 919
b0c22438 920This variable is read-only and dynamically-scoped.
f9cbb277 921
b0c22438 922Mnemonic: like C<&> in some editors.
0b9346e6 923
b0c22438 924=item ${^MATCH}
925X<${^MATCH}>
a0d0e21e 926
b0c22438 927This is similar to C<$&> (C<$MATCH>) except that it does not incur the
13b0f67d 928performance penalty associated with that variable.
40445027
DM
929
930See L</Performance issues> above.
931
13b0f67d 932In Perl v5.18 and earlier, it is only guaranteed
b0c22438 933to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
934the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
935C<${^MATCH}> does the same thing as C<$MATCH>.
80bca1b4 936
60cf4914 937This variable was added in Perl v5.10.0.
4bc88a62 938
b0c22438 939This variable is read-only and dynamically-scoped.
e2975953 940
b0c22438 941=item $PREMATCH
52c447a8 942
b0c22438 943=item $`
5b442a2a 944X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 945
b0c22438 946The string preceding whatever was matched by the last successful
947pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 948enclosed by the current BLOCK.
a0d0e21e 949
40445027
DM
950See L</Performance issues> above for the serious performance implications
951of using this variable (even once) in your code.
a0d0e21e 952
b0c22438 953This variable is read-only and dynamically-scoped.
a0d0e21e 954
b0c22438 955Mnemonic: C<`> often precedes a quoted string.
f83ed198 956
b0c22438 957=item ${^PREMATCH}
5b442a2a 958X<$`> X<${^PREMATCH}>
a0d0e21e 959
b0c22438 960This is similar to C<$`> ($PREMATCH) except that it does not incur the
13b0f67d 961performance penalty associated with that variable.
40445027
DM
962
963See L</Performance issues> above.
964
13b0f67d 965In Perl v5.18 and earlier, it is only guaranteed
b0c22438 966to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
967the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
968C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
a0d0e21e 969
4a70680a 970This variable was added in Perl v5.10.0.
a0d0e21e 971
b0c22438 972This variable is read-only and dynamically-scoped.
a0d0e21e 973
b0c22438 974=item $POSTMATCH
16070b82 975
b0c22438 976=item $'
5b442a2a 977X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 978
b0c22438 979The string following whatever was matched by the last successful
980pattern match (not counting any matches hidden within a BLOCK or C<eval()>
241a59d9 981enclosed by the current BLOCK). Example:
305aace0 982
9548c15c
FC
983 local $_ = 'abcdefghi';
984 /def/;
985 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 986
40445027
DM
987See L</Performance issues> above for the serious performance implications
988of using this variable (even once) in your code.
a0d0e21e 989
b0c22438 990This variable is read-only and dynamically-scoped.
991
992Mnemonic: C<'> often follows a quoted string.
993
994=item ${^POSTMATCH}
5b442a2a 995X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 996
997This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
13b0f67d 998performance penalty associated with that variable.
40445027
DM
999
1000See L</Performance issues> above.
1001
13b0f67d 1002In Perl v5.18 and earlier, it is only guaranteed
b0c22438 1003to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
1004the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1005C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
b0c22438 1006
60cf4914 1007This variable was added in Perl v5.10.0.
b0c22438 1008
1009This variable is read-only and dynamically-scoped.
1010
1011=item $LAST_PAREN_MATCH
1012
1013=item $+
1014X<$+> X<$LAST_PAREN_MATCH>
1015
1016The text matched by the last bracket of the last successful search pattern.
1017This is useful if you don't know which one of a set of alternative patterns
241a59d9 1018matched. For example:
b0c22438 1019
9548c15c 1020 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1021
1022This variable is read-only and dynamically-scoped.
1023
1024Mnemonic: be positive and forward looking.
1025
1026=item $LAST_SUBMATCH_RESULT
1027
1028=item $^N
5b442a2a 1029X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1030
1031The text matched by the used group most-recently closed (i.e. the group
1032with the rightmost closing parenthesis) of the last successful search
1033pattern.
1034
1035This is primarily used inside C<(?{...})> blocks for examining text
241a59d9 1036recently matched. For example, to effectively capture text to a variable
b0c22438 1037(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1038
9548c15c 1039 (?:(...)(?{ $var = $^N }))
b0c22438 1040
1041By setting and then using C<$var> in this way relieves you from having to
1042worry about exactly which numbered set of parentheses they are.
1043
60cf4914 1044This variable was added in Perl v5.8.0.
b0c22438 1045
1046Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1047
1048=item @LAST_MATCH_END
1049
1050=item @+
1051X<@+> X<@LAST_MATCH_END>
1052
1053This array holds the offsets of the ends of the last successful
241a59d9
FC
1054submatches in the currently active dynamic scope. C<$+[0]> is
1055the offset into the string of the end of the entire match. This
b0c22438 1056is the same value as what the C<pos> function returns when called
241a59d9 1057on the variable that was matched against. The I<n>th element
b0c22438 1058of this array holds the offset of the I<n>th submatch, so
1059C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
241a59d9
FC
1060past where C<$2> ends, and so on. You can use C<$#+> to determine
1061how many subgroups were in the last successful match. See the
b0c22438 1062examples given for the C<@-> variable.
1063
60cf4914 1064This variable was added in Perl v5.6.0.
b0c22438 1065
1066=item %LAST_PAREN_MATCH
1067
1068=item %+
5b442a2a 1069X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1070
1071Similar to C<@+>, the C<%+> hash allows access to the named capture
1072buffers, should they exist, in the last successful match in the
1073currently active dynamic scope.
1074
1075For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1076
9548c15c 1077 'foo' =~ /(?<foo>foo)/;
b0c22438 1078
1079The keys of the C<%+> hash list only the names of buffers that have
1080captured (and that are thus associated to defined values).
1081
1082The underlying behaviour of C<%+> is provided by the
1083L<Tie::Hash::NamedCapture> module.
1084
1085B<Note:> C<%-> and C<%+> are tied views into a common internal hash
241a59d9 1086associated with the last successful regular expression. Therefore mixing
b0c22438 1087iterative access to them via C<each> may have unpredictable results.
1088Likewise, if the last successful match changes, then the results may be
1089surprising.
1090
60cf4914 1091This variable was added in Perl v5.10.0.
a0d0e21e 1092
b0c22438 1093This variable is read-only and dynamically-scoped.
1094
1095=item @LAST_MATCH_START
1096
1097=item @-
1098X<@-> X<@LAST_MATCH_START>
1099
1100C<$-[0]> is the offset of the start of the last successful match.
1101C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1102I<n>-th subpattern, or undef if the subpattern did not match.
1103
1104Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
241a59d9 1105$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
b0c22438 1106$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
241a59d9
FC
1107C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1108last matched subgroup in the last successful match. Contrast with
1109C<$#+>, the number of subgroups in the regular expression. Compare
b0c22438 1110with C<@+>.
1111
1112This array holds the offsets of the beginnings of the last
1113successful submatches in the currently active dynamic scope.
1114C<$-[0]> is the offset into the string of the beginning of the
241a59d9 1115entire match. The I<n>th element of this array holds the offset
b0c22438 1116of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1117begins, C<$-[2]> the offset where C<$2> begins, and so on.
1118
1119After a match against some variable C<$var>:
1120
1121=over 5
1122
1123=item C<$`> is the same as C<substr($var, 0, $-[0])>
1124
1125=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1126
1127=item C<$'> is the same as C<substr($var, $+[0])>
1128
1129=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1130
1131=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1132
1133=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1134
1135=back
1136
60cf4914 1137This variable was added in Perl v5.6.0.
b0c22438 1138
1139=item %-
2e67aae7 1140X<%->
b0c22438 1141
1142Similar to C<%+>, this variable allows access to the named capture groups
241a59d9 1143in the last successful match in the currently active dynamic scope. To
b0c22438 1144each capture group name found in the regular expression, it associates a
1145reference to an array containing the list of values captured by all
1146buffers with that name (should there be several of them), in the order
1147where they appear.
1148
1149Here's an example:
1150
1151 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1152 foreach my $bufname (sort keys %-) {
1153 my $ary = $-{$bufname};
1154 foreach my $idx (0..$#$ary) {
1155 print "\$-{$bufname}[$idx] : ",
9548c15c
FC
1156 (defined($ary->[$idx])
1157 ? "'$ary->[$idx]'"
1158 : "undef"),
b0c22438 1159 "\n";
1160 }
1161 }
1162 }
1163
1164would print out:
1165
9548c15c
FC
1166 $-{A}[0] : '1'
1167 $-{A}[1] : '3'
1168 $-{B}[0] : '2'
1169 $-{B}[1] : '4'
b0c22438 1170
1171The keys of the C<%-> hash correspond to all buffer names found in
1172the regular expression.
1173
1174The behaviour of C<%-> is implemented via the
1175L<Tie::Hash::NamedCapture> module.
1176
1177B<Note:> C<%-> and C<%+> are tied views into a common internal hash
241a59d9 1178associated with the last successful regular expression. Therefore mixing
b0c22438 1179iterative access to them via C<each> may have unpredictable results.
1180Likewise, if the last successful match changes, then the results may be
1181surprising.
1182
60cf4914 1183This variable was added in Perl v5.10.0.
b0c22438 1184
1185This variable is read-only and dynamically-scoped.
1186
1187=item $LAST_REGEXP_CODE_RESULT
1188
1189=item $^R
1190X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1191
1192The result of evaluation of the last successful C<(?{ code })>
241a59d9 1193regular expression assertion (see L<perlre>). May be written to.
b0c22438 1194
1195This variable was added in Perl 5.005.
a0d0e21e 1196
a3621e74 1197=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1198X<${^RE_DEBUG_FLAGS}>
a3621e74 1199
241a59d9
FC
1200The current value of the regex debugging flags. Set to 0 for no debug output
1201even when the C<re 'debug'> module is loaded. See L<re> for details.
b0c22438 1202
60cf4914 1203This variable was added in Perl v5.10.0.
a3621e74 1204
0111c4fd 1205=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1206X<${^RE_TRIE_MAXBUF}>
a3621e74
YO
1207
1208Controls how certain regex optimisations are applied and how much memory they
241a59d9
FC
1209utilize. This value by default is 65536 which corresponds to a 512kB
1210temporary cache. Set this to a higher value to trade
1211memory for speed when matching large alternations. Set
1212it to a lower value if you want the optimisations to
a3621e74
YO
1213be as conservative of memory as possible but still occur, and set it to a
1214negative value to prevent the optimisation and conserve the most memory.
1215Under normal situations this variable should be of no interest to you.
1216
60cf4914 1217This variable was added in Perl v5.10.0.
a0d0e21e 1218
b0c22438 1219=back
a0d0e21e 1220
b0c22438 1221=head2 Variables related to filehandles
a0d0e21e 1222
b0c22438 1223Variables that depend on the currently selected filehandle may be set
1224by calling an appropriate object method on the C<IO::Handle> object,
1225although this is less efficient than using the regular built-in
241a59d9 1226variables. (Summary lines below for this contain the word HANDLE.)
b0c22438 1227First you must say
6e2995f4 1228
9548c15c 1229 use IO::Handle;
0462a1ab 1230
b0c22438 1231after which you may use either
0462a1ab 1232
9548c15c 1233 method HANDLE EXPR
0462a1ab 1234
b0c22438 1235or more safely,
0462a1ab 1236
9548c15c 1237 HANDLE->method(EXPR)
0462a1ab 1238
241a59d9 1239Each method returns the old value of the C<IO::Handle> attribute. The
b0c22438 1240methods each take an optional EXPR, which, if supplied, specifies the
241a59d9 1241new value for the C<IO::Handle> attribute in question. If not
b0c22438 1242supplied, most methods do nothing to the current value--except for
1243C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1244
b0c22438 1245Because loading in the C<IO::Handle> class is an expensive operation,
1246you should learn how to use the regular built-in variables.
1247
241a59d9 1248A few of these variables are considered "read-only". This means that
b0c22438 1249if you try to assign to this variable, either directly or indirectly
1250through a reference, you'll raise a run-time exception.
1251
1252You should be very careful when modifying the default values of most
241a59d9 1253special variables described in this document. In most cases you want
b0c22438 1254to localize these variables before changing them, since if you don't,
1255the change may affect other modules which rely on the default values
241a59d9 1256of the special variables that you have changed. This is one of the
b0c22438 1257correct ways to read the whole file at once:
1258
9548c15c
FC
1259 open my $fh, "<", "foo" or die $!;
1260 local $/; # enable localized slurp mode
1261 my $content = <$fh>;
1262 close $fh;
b0c22438 1263
1264But the following code is quite bad:
1265
9548c15c
FC
1266 open my $fh, "<", "foo" or die $!;
1267 undef $/; # enable slurp mode
1268 my $content = <$fh>;
1269 close $fh;
b0c22438 1270
1271since some other module, may want to read data from some file in the
1272default "line mode", so if the code we have just presented has been
1273executed, the global value of C<$/> is now changed for any other code
1274running inside the same Perl interpreter.
1275
1276Usually when a variable is localized you want to make sure that this
241a59d9
FC
1277change affects the shortest scope possible. So unless you are already
1278inside some short C<{}> block, you should create one yourself. For
b0c22438 1279example:
1280
9548c15c
FC
1281 my $content = '';
1282 open my $fh, "<", "foo" or die $!;
1283 {
1284 local $/;
1285 $content = <$fh>;
1286 }
1287 close $fh;
0462a1ab 1288
b0c22438 1289Here is an example of how your own code can go broken:
0462a1ab 1290
9548c15c
FC
1291 for ( 1..3 ){
1292 $\ = "\r\n";
1293 nasty_break();
1294 print "$_";
1295 }
0b9346e6 1296
9548c15c 1297 sub nasty_break {
0b9346e6 1298 $\ = "\f";
1299 # do something with $_
9548c15c 1300 }
0462a1ab 1301
0b9346e6 1302You probably expect this code to print the equivalent of
0462a1ab 1303
0b9346e6 1304 "1\r\n2\r\n3\r\n"
0462a1ab 1305
b0c22438 1306but instead you get:
0462a1ab 1307
0b9346e6 1308 "1\f2\f3\f"
0462a1ab 1309
0b9346e6 1310Why? Because C<nasty_break()> modifies C<$\> without localizing it
241a59d9
FC
1311first. The value you set in C<nasty_break()> is still there when you
1312return. The fix is to add C<local()> so the value doesn't leak out of
0b9346e6 1313C<nasty_break()>:
6e2995f4 1314
9548c15c 1315 local $\ = "\f";
a0d0e21e 1316
b0c22438 1317It's easy to notice the problem in such a short example, but in more
1318complicated code you are looking for trouble if you don't localize
1319changes to the special variables.
a0d0e21e 1320
b0c22438 1321=over 8
a0d0e21e 1322
b0c22438 1323=item $ARGV
1324X<$ARGV>
fb73857a 1325
ca1b95ae 1326Contains the name of the current file when reading from C<< <> >>.
b0c22438 1327
1328=item @ARGV
1329X<@ARGV>
1330
ca1b95ae 1331The array C<@ARGV> contains the command-line arguments intended for
241a59d9 1332the script. C<$#ARGV> is generally the number of arguments minus
b0c22438 1333one, because C<$ARGV[0]> is the first argument, I<not> the program's
241a59d9 1334command name itself. See L</$0> for the command name.
b0c22438 1335
84dabc03 1336=item ARGV
1337X<ARGV>
1338
1339The special filehandle that iterates over command-line filenames in
241a59d9
FC
1340C<@ARGV>. Usually written as the null filehandle in the angle operator
1341C<< <> >>. Note that currently C<ARGV> only has its magical effect
84dabc03 1342within the C<< <> >> operator; elsewhere it is just a plain filehandle
241a59d9 1343corresponding to the last file opened by C<< <> >>. In particular,
84dabc03 1344passing C<\*ARGV> as a parameter to a function that expects a filehandle
1345may not cause your function to automatically read the contents of all the
1346files in C<@ARGV>.
1347
b0c22438 1348=item ARGVOUT
1349X<ARGVOUT>
1350
1351The special filehandle that points to the currently open output file
241a59d9
FC
1352when doing edit-in-place processing with B<-i>. Useful when you have
1353to do a lot of inserting and don't want to keep modifying C<$_>. See
b0c22438 1354L<perlrun> for the B<-i> switch.
1355
96948506 1356=item IO::Handle->output_field_separator( EXPR )
84dabc03 1357
1358=item $OUTPUT_FIELD_SEPARATOR
1359
1360=item $OFS
1361
1362=item $,
1363X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1364
241a59d9
FC
1365The output field separator for the print operator. If defined, this
1366value is printed between each of print's arguments. Default is C<undef>.
84dabc03 1367
96948506 1368You cannot call C<output_field_separator()> on a handle, only as a
008f9687 1369static method. See L<IO::Handle|IO::Handle>.
96948506 1370
84dabc03 1371Mnemonic: what is printed when there is a "," in your print statement.
1372
5b442a2a 1373=item HANDLE->input_line_number( EXPR )
b0c22438 1374
1375=item $INPUT_LINE_NUMBER
1376
1377=item $NR
1378
1379=item $.
1380X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1381
1382Current line number for the last filehandle accessed.
1383
1384Each filehandle in Perl counts the number of lines that have been read
241a59d9 1385from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1386constitutes a line may not match yours.) When a line is read from a
1387filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1388C<seek()> is called on it, C<$.> becomes an alias to the line counter
1389for that filehandle.
1390
1391You can adjust the counter by assigning to C<$.>, but this will not
241a59d9
FC
1392actually move the seek pointer. I<Localizing C<$.> will not localize
1393the filehandle's line count>. Instead, it will localize perl's notion
b0c22438 1394of which filehandle C<$.> is currently aliased to.
1395
1396C<$.> is reset when the filehandle is closed, but B<not> when an open
241a59d9
FC
1397filehandle is reopened without an intervening C<close()>. For more
1398details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
b0c22438 1399an explicit close, line numbers increase across C<ARGV> files (but see
1400examples in L<perlfunc/eof>).
1401
1402You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1403line counter for a given filehandle without having to worry about
1404which handle you last accessed.
1405
1406Mnemonic: many programs use "." to mean the current line number.
1407
96948506 1408=item IO::Handle->input_record_separator( EXPR )
b0c22438 1409
1410=item $INPUT_RECORD_SEPARATOR
1411
1412=item $RS
1413
1414=item $/
1415X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1416
241a59d9
FC
1417The input record separator, newline by default. This influences Perl's
1418idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1419treating empty lines as a terminator if set to the null string (an
241a59d9 1420empty line cannot contain any spaces or tabs). You may set it to a
84dabc03 1421multi-character string to match a multi-character terminator, or to
241a59d9 1422C<undef> to read through the end of file. Setting it to C<"\n\n">
84dabc03 1423means something slightly different than setting to C<"">, if the file
241a59d9
FC
1424contains consecutive empty lines. Setting to C<""> will treat two or
1425more consecutive empty lines as a single empty line. Setting to
84dabc03 1426C<"\n\n"> will blindly assume that the next input character belongs to
1427the next paragraph, even if it's a newline.
b0c22438 1428
1429 local $/; # enable "slurp" mode
1430 local $_ = <FH>; # whole file now here
1431 s/\n[ \t]+/ /g;
1432
241a59d9 1433Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1434be better for something. :-)
1435
1436Setting C<$/> to a reference to an integer, scalar containing an
1437integer, or scalar that's convertible to an integer will attempt to
1438read records instead of lines, with the maximum record size being the
3d249121 1439referenced integer number of characters. So this:
b0c22438 1440
1441 local $/ = \32768; # or \"32768", or \$var_containing_32768
1442 open my $fh, "<", $myfile or die $!;
1443 local $_ = <$fh>;
fb73857a 1444
f1ee460b 1445will read a record of no more than 32768 characters from $fh. If you're
b0c22438 1446not reading from a record-oriented file (or your OS doesn't have
1447record-oriented files), then you'll likely get a full chunk of data
241a59d9
FC
1448with every read. If a record is larger than the record size you've
1449set, you'll get the record back in pieces. Trying to set the record
b3a2acfa
YO
1450size to zero or less is deprecated and will cause $/ to have the value
1451of "undef", which will cause reading in the (rest of the) whole file.
1452
1453As of 5.19.9 setting C<$/> to any other form of reference will throw a
1454fatal exception. This is in preparation for supporting new ways to set
1455C<$/> in the future.
6e2995f4 1456
78c28381 1457On VMS only, record reads bypass PerlIO layers and any associated
3d249121 1458buffering, so you must not mix record and non-record reads on the
78c28381
CB
1459same filehandle. Record mode mixes with line mode only when the
1460same buffering layer is in use for both modes.
5c055ba3 1461
96948506 1462You cannot call C<input_record_separator()> on a handle, only as a
008f9687 1463static method. See L<IO::Handle|IO::Handle>.
96948506 1464
008f9687 1465See also L<perlport/"Newlines">. Also see L</$.>.
9bf22702 1466
b0c22438 1467Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1468
96948506 1469=item IO::Handle->output_record_separator( EXPR )
84902520 1470
b0c22438 1471=item $OUTPUT_RECORD_SEPARATOR
84902520 1472
b0c22438 1473=item $ORS
84902520 1474
b0c22438 1475=item $\
1476X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
84902520 1477
241a59d9
FC
1478The output record separator for the print operator. If defined, this
1479value is printed after the last of print's arguments. Default is C<undef>.
84902520 1480
96948506 1481You cannot call C<output_record_separator()> on a handle, only as a
008f9687 1482static method. See L<IO::Handle|IO::Handle>.
96948506 1483
b0c22438 1484Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1485Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1486
5b442a2a 1487=item HANDLE->autoflush( EXPR )
1488
1489=item $OUTPUT_AUTOFLUSH
1490
84dabc03 1491=item $|
1492X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1493
84dabc03 1494If set to nonzero, forces a flush right away and after every write or
241a59d9 1495print on the currently selected output channel. Default is 0
84dabc03 1496(regardless of whether the channel is really buffered by the system or
1497not; C<$|> tells you only whether you've asked Perl explicitly to
241a59d9
FC
1498flush after each write). STDOUT will typically be line buffered if
1499output is to the terminal and block buffered otherwise. Setting this
84dabc03 1500variable is useful primarily when you are outputting to a pipe or
1501socket, such as when you are running a Perl program under B<rsh> and
241a59d9
FC
1502want to see the output as it's happening. This has no effect on input
1503buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1504how to select the output channel. See also L<IO::Handle>.
84dabc03 1505
1506Mnemonic: when you want your pipes to be piping hot.
1507
8561ea1d
FC
1508=item ${^LAST_FH}
1509X<${^LAST_FH}>
1510
1511This read-only variable contains a reference to the last-read filehandle.
1512This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1513This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1514use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1515an error or warning message.
1516
1517This variable was added in Perl v5.18.0.
1518
84dabc03 1519=back
84902520 1520
b0c22438 1521=head3 Variables related to formats
83ee9e09 1522
b0c22438 1523The special variables for formats are a subset of those for
241a59d9 1524filehandles. See L<perlform> for more information about Perl's
69b55ccc 1525formats.
83ee9e09 1526
b0c22438 1527=over 8
83ee9e09 1528
84dabc03 1529=item $ACCUMULATOR
1530
1531=item $^A
1532X<$^A> X<$ACCUMULATOR>
1533
1534The current value of the C<write()> accumulator for C<format()> lines.
1535A format contains C<formline()> calls that put their result into
241a59d9
FC
1536C<$^A>. After calling its format, C<write()> prints out the contents
1537of C<$^A> and empties. So you never really see the contents of C<$^A>
1538unless you call C<formline()> yourself and then look at it. See
96090e4f 1539L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
84dabc03 1540
96948506 1541=item IO::Handle->format_formfeed(EXPR)
5b442a2a 1542
1543=item $FORMAT_FORMFEED
1544
84dabc03 1545=item $^L
1546X<$^L> X<$FORMAT_FORMFEED>
1547
241a59d9 1548What formats output as a form feed. The default is C<\f>.
84dabc03 1549
96948506 1550You cannot call C<format_formfeed()> on a handle, only as a static
008f9687 1551method. See L<IO::Handle|IO::Handle>.
96948506 1552
b0c22438 1553=item HANDLE->format_page_number(EXPR)
83ee9e09 1554
b0c22438 1555=item $FORMAT_PAGE_NUMBER
83ee9e09 1556
b0c22438 1557=item $%
1558X<$%> X<$FORMAT_PAGE_NUMBER>
83ee9e09 1559
b0c22438 1560The current page number of the currently selected output channel.
83ee9e09 1561
b0c22438 1562Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1563
b0c22438 1564=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1565
b0c22438 1566=item $FORMAT_LINES_LEFT
66558a10 1567
b0c22438 1568=item $-
1569X<$-> X<$FORMAT_LINES_LEFT>
fb73857a 1570
b0c22438 1571The number of lines left on the page of the currently selected output
1572channel.
fa05a9fd 1573
b0c22438 1574Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1575
96948506 1576=item IO::Handle->format_line_break_characters EXPR
fb73857a 1577
84dabc03 1578=item $FORMAT_LINE_BREAK_CHARACTERS
a0d0e21e 1579
84dabc03 1580=item $:
1581X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e 1582
84dabc03 1583The current set of characters after which a string may be broken to
241a59d9 1584fill continuation fields (starting with C<^>) in a format. The default is
84dabc03 1585S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1586
96948506 1587You cannot call C<format_line_break_characters()> on a handle, only as
008f9687 1588a static method. See L<IO::Handle|IO::Handle>.
96948506 1589
84dabc03 1590Mnemonic: a "colon" in poetry is a part of a line.
1591
1592=item HANDLE->format_lines_per_page(EXPR)
1593
1594=item $FORMAT_LINES_PER_PAGE
1595
1596=item $=
1597X<$=> X<$FORMAT_LINES_PER_PAGE>
1598
1599The current page length (printable lines) of the currently selected
241a59d9 1600output channel. The default is 60.
84dabc03 1601
1602Mnemonic: = has horizontal lines.
7c36658b 1603
b0c22438 1604=item HANDLE->format_top_name(EXPR)
7c36658b 1605
b0c22438 1606=item $FORMAT_TOP_NAME
a05d7ebb 1607
b0c22438 1608=item $^
1609X<$^> X<$FORMAT_TOP_NAME>
fde18df1 1610
b0c22438 1611The name of the current top-of-page format for the currently selected
241a59d9
FC
1612output channel. The default is the name of the filehandle with C<_TOP>
1613appended. For example, the default format top name for the C<STDOUT>
12abbafd 1614filehandle is C<STDOUT_TOP>.
e07ea26a 1615
b0c22438 1616Mnemonic: points to top of page.
e07ea26a 1617
84dabc03 1618=item HANDLE->format_name(EXPR)
16070b82 1619
84dabc03 1620=item $FORMAT_NAME
aa2f2a36 1621
84dabc03 1622=item $~
1623X<$~> X<$FORMAT_NAME>
aa2f2a36 1624
84dabc03 1625The name of the current report format for the currently selected
241a59d9
FC
1626output channel. The default format name is the same as the filehandle
1627name. For example, the default format name for the C<STDOUT>
84dabc03 1628filehandle is just C<STDOUT>.
16070b82 1629
84dabc03 1630Mnemonic: brother to C<$^>.
16070b82 1631
b0c22438 1632=back
a0d0e21e 1633
84dabc03 1634=head2 Error Variables
b0c22438 1635X<error> X<exception>
a0d0e21e 1636
b0c22438 1637The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1638about different types of error conditions that may appear during
241a59d9 1639execution of a Perl program. The variables are shown ordered by
b0c22438 1640the "distance" between the subsystem which reported the error and
241a59d9 1641the Perl process. They correspond to errors detected by the Perl
b0c22438 1642interpreter, C library, operating system, or an external program,
1643respectively.
4438c4b7 1644
b0c22438 1645To illustrate the differences between these variables, consider the
241a59d9 1646following Perl expression, which uses a single-quoted string. After
7fd683ff 1647execution of this statement, perl may have set all four special error
7333b1c4 1648variables:
4438c4b7 1649
9548c15c
FC
1650 eval q{
1651 open my $pipe, "/cdrom/install |" or die $!;
1652 my @res = <$pipe>;
1653 close $pipe or die "bad pipe: $?, $!";
1654 };
a0d0e21e 1655
7333b1c4 1656When perl executes the C<eval()> expression, it translates the
1657C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
241a59d9 1658and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1659the C library's C<errno> if one of these calls fails.
2a8c8378 1660
84dabc03 1661C<$@> is set if the string to be C<eval>-ed did not compile (this may
1662happen if C<open> or C<close> were imported with bad prototypes), or
241a59d9 1663if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1664value of C<$@> is the compile error, or the argument to C<die> (which
241a59d9 1665will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1666
84dabc03 1667Under a few operating systems, C<$^E> may contain a more verbose error
241a59d9 1668indicator, such as in this case, "CDROM tray not closed." Systems that
84dabc03 1669do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1670
2e6ba115 1671Finally, C<$?> may be set to a non-0 value if the external program
241a59d9 1672F</cdrom/install> fails. The upper eight bits reflect specific error
84dabc03 1673conditions encountered by the program (the program's C<exit()> value).
1674The lower eight bits reflect mode of failure, like signal death and
241a59d9 1675core dump information. See L<wait(2)> for details. In contrast to
2e6ba115 1676C<$!> and C<$^E>, which are set only if an error condition is detected,
84dabc03 1677the variable C<$?> is set on each C<wait> or pipe C<close>,
241a59d9 1678overwriting the old value. This is more like C<$@>, which on every
84dabc03 1679C<eval()> is always set on failure and cleared on success.
a0d0e21e 1680
b0c22438 1681For more details, see the individual descriptions at C<$@>, C<$!>,
1682C<$^E>, and C<$?>.
38e4f4ae 1683
0b9346e6 1684=over 8
1685
b0c22438 1686=item ${^CHILD_ERROR_NATIVE}
1687X<$^CHILD_ERROR_NATIVE>
a0d0e21e 1688
b0c22438 1689The native status returned by the last pipe close, backtick (C<``>)
1690command, successful call to C<wait()> or C<waitpid()>, or from the
241a59d9 1691C<system()> operator. On POSIX-like systems this value can be decoded
b0c22438 1692with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1693WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1694
b0c22438 1695Under VMS this reflects the actual VMS exit status; i.e. it is the
1696same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1697
60cf4914 1698This variable was added in Perl v5.10.0.
a0d0e21e 1699
5b442a2a 1700=item $EXTENDED_OS_ERROR
1701
84dabc03 1702=item $^E
1703X<$^E> X<$EXTENDED_OS_ERROR>
1704
241a59d9 1705Error information specific to the current operating system. At the
a804e657 1706moment, this differs from C<L</$!>> under only VMS, OS/2, and Win32 (and
241a59d9 1707for MacPerl). On all other platforms, C<$^E> is always just the same
84dabc03 1708as C<$!>.
1709
1710Under VMS, C<$^E> provides the VMS status value from the last system
241a59d9
FC
1711error. This is more specific information about the last system error
1712than that provided by C<$!>. This is particularly important when C<$!>
84dabc03 1713is set to B<EVMSERR>.
1714
1715Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1716API either via CRT, or directly from perl.
1717
1718Under Win32, C<$^E> always returns the last error information reported
1719by the Win32 call C<GetLastError()> which describes the last error
241a59d9
FC
1720from within the Win32 API. Most Win32-specific code will report errors
1721via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
84dabc03 1722portable Perl code will report errors via C<$!>.
1723
a95b3d6a 1724Caveats mentioned in the description of C<L</$!>> generally apply to
84dabc03 1725C<$^E>, also.
1726
1727This variable was added in Perl 5.003.
1728
1729Mnemonic: Extra error explanation.
0b9346e6 1730
84dabc03 1731=item $EXCEPTIONS_BEING_CAUGHT
1732
1733=item $^S
1734X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1735
1736Current state of the interpreter.
1737
ca1b95ae 1738 $^S State
aa959a20
FC
1739 --------- -------------------------------------
1740 undef Parsing module, eval, or main program
ca1b95ae 1741 true (1) Executing an eval
1742 false (0) Otherwise
84dabc03 1743
1744The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1745handlers.
1746
aa959a20
FC
1747The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1748the C<undef> value does not indicate whether exceptions are being caught,
1749since compilation of the main program does not catch exceptions.
1750
84dabc03 1751This variable was added in Perl 5.004.
1752
1753=item $WARNING
1754
1755=item $^W
1756X<$^W> X<$WARNING>
1757
1758The current value of the warning switch, initially true if B<-w> was
1759used, false otherwise, but directly modifiable.
1760
1761See also L<warnings>.
1762
0b9346e6 1763Mnemonic: related to the B<-w> switch.
84dabc03 1764
1765=item ${^WARNING_BITS}
ca1b95ae 1766X<${^WARNING_BITS}>
84dabc03 1767
1768The current set of warning checks enabled by the C<use warnings> pragma.
44567c86
FC
1769It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1770values are considered internal to the L<warnings> pragma and may change
1771between versions of Perl.
84dabc03 1772
60cf4914 1773This variable was added in Perl v5.6.0.
84dabc03 1774
b0c22438 1775=item $OS_ERROR
5ccee41e 1776
b0c22438 1777=item $ERRNO
5ccee41e 1778
b0c22438 1779=item $!
1780X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1781
a73bef78
JL
1782When referenced, C<$!> retrieves the current value
1783of the C C<errno> integer variable.
1784If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1785When referenced as a string, C<$!> yields the system error string
1786corresponding to C<errno>.
1787
1788Many system or library calls set C<errno> if they fail,
1789to indicate the cause of failure. They usually do B<not>
1790set C<errno> to zero if they succeed. This means C<errno>,
1791hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1792
1793 if (open my $fh, "<", $filename) {
ca1b95ae 1794 # Here $! is meaningless.
1795 ...
7fd683ff 1796 }
ca1b95ae 1797 else {
1798 # ONLY here is $! meaningful.
1799 ...
1800 # Already here $! might be meaningless.
b0c22438 1801 }
1802 # Since here we might have either success or failure,
a73bef78 1803 # $! is meaningless.
a0d0e21e 1804
a73bef78
JL
1805Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1806of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1807It can be used immediately before invoking the C<die()> operator,
1808to set the exit value, or to inspect the system error string
1809corresponding to error I<n>, or to restore C<$!> to a meaningful state.
d54b56d5 1810
b0c22438 1811Mnemonic: What just went bang?
314d39ce 1812
b0c22438 1813=item %OS_ERROR
fb73857a 1814
b0c22438 1815=item %ERRNO
fb73857a 1816
b0c22438 1817=item %!
5b442a2a 1818X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1819
b0c22438 1820Each element of C<%!> has a true value only if C<$!> is set to that
241a59d9 1821value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1822value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1823such file or directory" (or its moral equivalent: not all operating
3b90fd91
RS
1824systems give that exact error, and certainly not all languages). The
1825specific true value is not guaranteed, but in the past has generally
1826been the numeric value of C<$!>. To check if a particular key is
1827meaningful on your system, use C<exists $!{the_key}>; for a list of legal
1828keys, use C<keys %!>. See L<Errno> for more information, and also see
1829L</$!>.
a0d0e21e 1830
b0c22438 1831This variable was added in Perl 5.005.
44f0be63 1832
84dabc03 1833=item $CHILD_ERROR
b687b08b 1834
84dabc03 1835=item $?
1836X<$?> X<$CHILD_ERROR>
a0d0e21e 1837
84dabc03 1838The status returned by the last pipe close, backtick (C<``>) command,
1839successful call to C<wait()> or C<waitpid()>, or from the C<system()>
241a59d9 1840operator. This is just the 16-bit status word returned by the
84dabc03 1841traditional Unix C<wait()> system call (or else is made up to look
241a59d9 1842like it). Thus, the exit value of the subprocess is really (C<<< $? >>
84dabc03 18438 >>>), and C<$? & 127> gives which signal, if any, the process died
1844from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1845
84dabc03 1846Additionally, if the C<h_errno> variable is supported in C, its value
1847is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1848
84dabc03 1849If you have installed a signal handler for C<SIGCHLD>, the
1850value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1851
84dabc03 1852Inside an C<END> subroutine C<$?> contains the value that is going to be
241a59d9
FC
1853given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1854change the exit status of your program. For example:
a0d0e21e 1855
84dabc03 1856 END {
1857 $? = 1 if $? == 255; # die would make it 255
1858 }
a0d0e21e 1859
84dabc03 1860Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1861actual VMS exit status, instead of the default emulation of POSIX
1862status; see L<perlvms/$?> for details.
1863
1864Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1865
b0c22438 1866=item $EVAL_ERROR
f648820c 1867
b0c22438 1868=item $@
1869X<$@> X<$EVAL_ERROR>
a0d0e21e 1870
2e6ba115
LM
1871The Perl error from the last C<eval> operator, i.e. the last exception that
1872was caught. For C<eval BLOCK>, this is either a runtime error message or the
1873string or reference C<die> was called with. The C<eval STRING> form also
1874catches syntax errors and other compile time exceptions.
1875
1876If no error occurs, C<eval> sets C<$@> to the empty string.
a0d0e21e 1877
241a59d9 1878Warning messages are not collected in this variable. You can, however,
b0c22438 1879set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 1880described in L</%SIG>.
748a9306 1881
2e6ba115 1882Mnemonic: Where was the error "at"?
7f315d2e 1883
b0c22438 1884=back
7f315d2e 1885
1fa81471
DR
1886=head2 Variables related to the interpreter state
1887
1888These variables provide information about the current interpreter state.
1889
1890=over 8
1891
1892=item $COMPILING
1893
1894=item $^C
1895X<$^C> X<$COMPILING>
1896
1897The current value of the flag associated with the B<-c> switch.
1898Mainly of use with B<-MO=...> to allow code to alter its behavior
1899when being compiled, such as for example to C<AUTOLOAD> at compile
241a59d9 1900time rather than normal, deferred loading. Setting
1fa81471
DR
1901C<$^C = 1> is similar to calling C<B::minus_c>.
1902
60cf4914 1903This variable was added in Perl v5.6.0.
1fa81471
DR
1904
1905=item $DEBUGGING
1906
1907=item $^D
1908X<$^D> X<$DEBUGGING>
1909
241a59d9 1910The current value of the debugging flags. May be read or set. Like its
a975eeba 1911L<command-line equivalent|perlrun/B<-D>I<letters>>, you can use numeric
8af589bf
KW
1912or symbolic values, e.g. C<$^D = 10> or C<$^D = "st">. See
1913L<perlrun/B<-D>I<number>>. The contents of this variable also affects the
1914debugger operation. See L<perldebguts/Debugger Internals>.
1fa81471
DR
1915
1916Mnemonic: value of B<-D> switch.
1917
1918=item ${^ENCODING}
1919X<${^ENCODING}>
1920
981b911e 1921This variable is no longer supported.
a3ee04ba 1922
981b911e
FC
1923It used to hold the I<object reference> to the C<Encode> object that was
1924used to convert the source code to Unicode.
a3ee04ba 1925
981b911e
FC
1926Its purpose was to allow your non-ASCII Perl
1927scripts not to have to be written in UTF-8; this was
a3ee04ba 1928useful before editors that worked on UTF-8 encoded text were common, but
981b911e
FC
1929that was long ago. It caused problems, such as affecting the operation
1930of other modules that weren't expecting it, causing general mayhem.
a3ee04ba 1931
981b911e
FC
1932If you need something like this functionality, it is recommended that use
1933you a simple source filter, such as L<Filter::Encoding>.
a3ee04ba
KW
1934
1935If you are coming here because code of yours is being adversely affected
1936by someone's use of this variable, you can usually work around it by
1937doing this:
1938
1939 local ${^ENCODING};
1940
1941near the beginning of the functions that are getting broken. This
1942undefines the variable during the scope of execution of the including
1943function.
1fa81471 1944
981b911e 1945This variable was added in Perl 5.8.2 and removed in 5.26.0.
1fa81471
DR
1946
1947=item ${^GLOBAL_PHASE}
1948X<${^GLOBAL_PHASE}>
1949
1950The current phase of the perl interpreter.
1951
1952Possible values are:
1953
1954=over 8
1955
1956=item CONSTRUCT
1957
241a59d9 1958The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1fa81471 1959value is mostly there for completeness and for use via the
241a59d9 1960underlying C variable C<PL_phase>. It's not really possible for Perl
1fa81471
DR
1961code to be executed unless construction of the interpreter is
1962finished.
1963
1964=item START
1965
241a59d9 1966This is the global compile-time. That includes, basically, every
1fa81471
DR
1967C<BEGIN> block executed directly or indirectly from during the
1968compile-time of the top-level program.
1969
1970This phase is not called "BEGIN" to avoid confusion with
1971C<BEGIN>-blocks, as those are executed during compile-time of any
241a59d9 1972compilation unit, not just the top-level program. A new, localised
1fa81471
DR
1973compile-time entered at run-time, for example by constructs as
1974C<eval "use SomeModule"> are not global interpreter phases, and
1975therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1976
1977=item CHECK
1978
1979Execution of any C<CHECK> blocks.
1980
1981=item INIT
1982
1983Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1984
1985=item RUN
1986
1987The main run-time, i.e. the execution of C<PL_main_root>.
1988
1989=item END
1990
1991Execution of any C<END> blocks.
1992
1993=item DESTRUCT
1994
1995Global destruction.
1996
1997=back
1998
241a59d9 1999Also note that there's no value for UNITCHECK-blocks. That's because
1fa81471
DR
2000those are run for each compilation unit individually, and therefore is
2001not a global interpreter phase.
2002
2003Not every program has to go through each of the possible phases, but
2004transition from one phase to another can only happen in the order
2005described in the above list.
2006
2007An example of all of the phases Perl code can see:
2008
2009 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
2010
2011 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
2012
2013 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
2014
2015 {
2016 package Print::Phase;
2017
2018 sub new {
2019 my ($class, $time) = @_;
2020 return bless \$time, $class;
2021 }
2022
2023 sub DESTROY {
2024 my $self = shift;
2025 print "$$self: ${^GLOBAL_PHASE}\n";
2026 }
2027 }
2028
2029 print "run-time: ${^GLOBAL_PHASE}\n";
2030
2031 my $runtime = Print::Phase->new(
2032 "lexical variables are garbage collected before END"
2033 );
2034
2035 END { print "end-time: ${^GLOBAL_PHASE}\n" }
2036
2037 our $destruct = Print::Phase->new(
2038 "package variables are garbage collected after END"
2039 );
2040
2041This will print out
2042
2043 compile-time: START
2044 check-time: CHECK
2045 init-time: INIT
2046 run-time: RUN
2047 lexical variables are garbage collected before END: RUN
2048 end-time: END
2049 package variables are garbage collected after END: DESTRUCT
2050
2051This variable was added in Perl 5.14.0.
2052
2053=item $^H
2054X<$^H>
2055
241a59d9
FC
2056WARNING: This variable is strictly for
2057internal use only. Its availability,
1fa81471
DR
2058behavior, and contents are subject to change without notice.
2059
241a59d9 2060This variable contains compile-time hints for the Perl interpreter. At the
1fa81471
DR
2061end of compilation of a BLOCK the value of this variable is restored to the
2062value when the interpreter started to compile the BLOCK.
2063
2064When perl begins to parse any block construct that provides a lexical scope
2065(e.g., eval body, required file, subroutine body, loop body, or conditional
2066block), the existing value of C<$^H> is saved, but its value is left unchanged.
2067When the compilation of the block is completed, it regains the saved value.
2068Between the points where its value is saved and restored, code that
2069executes within BEGIN blocks is free to change the value of C<$^H>.
2070
2071This behavior provides the semantic of lexical scoping, and is used in,
2072for instance, the C<use strict> pragma.
2073
2074The contents should be an integer; different bits of it are used for
241a59d9 2075different pragmatic flags. Here's an example:
1fa81471 2076
9548c15c 2077 sub add_100 { $^H |= 0x100 }
1fa81471 2078
9548c15c
FC
2079 sub foo {
2080 BEGIN { add_100() }
2081 bar->baz($boon);
2082 }
1fa81471 2083
241a59d9 2084Consider what happens during execution of the BEGIN block. At this point
1fa81471 2085the BEGIN block has already been compiled, but the body of C<foo()> is still
241a59d9
FC
2086being compiled. The new value of C<$^H>
2087will therefore be visible only while
1fa81471
DR
2088the body of C<foo()> is being compiled.
2089
2090Substitution of C<BEGIN { add_100() }> block with:
2091
9548c15c 2092 BEGIN { require strict; strict->import('vars') }
1fa81471 2093
241a59d9 2094demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1fa81471
DR
2095version of the same lexical pragma:
2096
9548c15c
FC
2097 BEGIN {
2098 require strict; strict->import('vars') if $condition
2099 }
1fa81471
DR
2100
2101This variable was added in Perl 5.003.
2102
2103=item %^H
2104X<%^H>
2105
241a59d9
FC
2106The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2107it useful for implementation of lexically scoped pragmas. See
112284f4
KW
2108L<perlpragma>. All the entries are stringified when accessed at
2109runtime, so only simple values can be accommodated. This means no
2110pointers to objects, for example.
1fa81471
DR
2111
2112When putting items into C<%^H>, in order to avoid conflicting with other
2113users of the hash there is a convention regarding which keys to use.
2114A module should use only keys that begin with the module's name (the
2115name of its main package) and a "/" character. For example, a module
2116C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2117
60cf4914 2118This variable was added in Perl v5.6.0.
1fa81471
DR
2119
2120=item ${^OPEN}
2121X<${^OPEN}>
2122
241a59d9 2123An internal variable used by PerlIO. A string in two parts, separated
1fa81471
DR
2124by a C<\0> byte, the first part describes the input layers, the second
2125part describes the output layers.
2126
60cf4914 2127This variable was added in Perl v5.8.0.
1fa81471
DR
2128
2129=item $PERLDB
2130
2131=item $^P
2132X<$^P> X<$PERLDB>
2133
241a59d9 2134The internal variable for debugging support. The meanings of the
1fa81471
DR
2135various bits are subject to change, but currently indicate:
2136
2137=over 6
2138
2139=item 0x01
2140
2141Debug subroutine enter/exit.
2142
2143=item 0x02
2144
241a59d9
FC
2145Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2146each statement executed. Also causes saving source code lines (like
21470x400).
1fa81471
DR
2148
2149=item 0x04
2150
2151Switch off optimizations.
2152
2153=item 0x08
2154
2155Preserve more data for future interactive inspections.
2156
2157=item 0x10
2158
2159Keep info about source lines on which a subroutine is defined.
2160
2161=item 0x20
2162
2163Start with single-step on.
2164
2165=item 0x40
2166
2167Use subroutine address instead of name when reporting.
2168
2169=item 0x80
2170
2171Report C<goto &subroutine> as well.
2172
2173=item 0x100
2174
2175Provide informative "file" names for evals based on the place they were compiled.
2176
2177=item 0x200
2178
2179Provide informative names to anonymous subroutines based on the place they
2180were compiled.
2181
2182=item 0x400
2183
2184Save source code lines into C<@{"_<$filename"}>.
2185
aab47982
RS
2186=item 0x800
2187
2188When saving source, include evals that generate no subroutines.
2189
2190=item 0x1000
2191
2192When saving source, include source that did not compile.
2193
1fa81471
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2194=back
2195
2196Some bits may be relevant at compile-time only, some at
241a59d9 2197run-time only. This is a new mechanism and the details may change.
1fa81471
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2198See also L<perldebguts>.
2199
2200=item ${^TAINT}
2201X<${^TAINT}>
2202
241a59d9 2203Reflects if taint mode is on or off. 1 for on (the program was run with
1fa81471
DR
2204B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2205B<-t> or B<-TU>).
2206
2207This variable is read-only.
2208
60cf4914 2209This variable was added in Perl v5.8.0.
1fa81471
DR
2210
2211=item ${^UNICODE}
2212X<${^UNICODE}>
2213
241a59d9 2214Reflects certain Unicode settings of Perl. See L<perlrun>
1fa81471
DR
2215documentation for the C<-C> switch for more information about
2216the possible values.
2217
2218This variable is set during Perl startup and is thereafter read-only.
2219
60cf4914 2220This variable was added in Perl v5.8.2.
1fa81471
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2221
2222=item ${^UTF8CACHE}
2223X<${^UTF8CACHE}>
2224
2225This variable controls the state of the internal UTF-8 offset caching code.
22261 for on (the default), 0 for off, -1 to debug the caching code by checking
2227all its results against linear scans, and panicking on any discrepancy.
2228
94df5432
KW
2229This variable was added in Perl v5.8.9. It is subject to change or
2230removal without notice, but is currently used to avoid recalculating the
2231boundaries of multi-byte UTF-8-encoded characters.
1fa81471
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2232
2233=item ${^UTF8LOCALE}
2234X<${^UTF8LOCALE}>
2235
2236This variable indicates whether a UTF-8 locale was detected by perl at
241a59d9 2237startup. This information is used by perl when it's in
1fa81471
DR
2238adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2239switch); see L<perlrun> for more info on this.
2240
60cf4914 2241This variable was added in Perl v5.8.8.
1fa81471
DR
2242
2243=back
2244
b0c22438 2245=head2 Deprecated and removed variables
7f315d2e 2246
0b9346e6 2247Deprecating a variable announces the intent of the perl maintainers to
241a59d9
FC
2248eventually remove the variable from the language. It may still be
2249available despite its status. Using a deprecated variable triggers
b0c22438 2250a warning.
7f315d2e 2251
84dabc03 2252Once a variable is removed, its use triggers an error telling you
b0c22438 2253the variable is unsupported.
7f315d2e 2254
84dabc03 2255See L<perldiag> for details about error messages.
7f315d2e 2256
b0c22438 2257=over 8
7f315d2e 2258
84dabc03 2259=item $#
b7a15f05 2260X<$#>
84dabc03 2261
38e5787b 2262C<$#> was a variable that could be used to format printed numbers.
60cf4914 2263After a deprecation cycle, its magic was removed in Perl v5.10.0 and
84dabc03 2264using it now triggers a warning: C<$# is no longer supported>.
2265
2266This is not the sigil you use in front of an array name to get the
241a59d9
FC
2267last index, like C<$#array>. That's still how you get the last index
2268of an array in Perl. The two have nothing to do with each other.
84dabc03 2269
2270Deprecated in Perl 5.
2271
60cf4914 2272Removed in Perl v5.10.0.
84dabc03 2273
7f315d2e
CO
2274=item $*
2275X<$*>
2276
84dabc03 2277C<$*> was a variable that you could use to enable multiline matching.
60cf4914 2278After a deprecation cycle, its magic was removed in Perl v5.10.0.
7f315d2e 2279Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2280You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2281
b0c22438 2282Deprecated in Perl 5.
7f315d2e 2283
60cf4914 2284Removed in Perl v5.10.0.
7f315d2e 2285
84dabc03 2286=item $[
b7a15f05 2287X<$[>
84dabc03 2288
b82b06b8
FC
2289This variable stores the index of the first element in an array, and
2290of the first character in a substring. The default is 0, but you could
2291theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2292when subscripting and when evaluating the index() and substr() functions.
84dabc03 2293
b82b06b8
FC
2294As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2295directive, and cannot influence the behavior of any other file.
2296(That's why you can only assign compile-time constants to it.)
2297Its use is highly discouraged.
2298
60cf4914 2299Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
b82b06b8
FC
2300scopes in the same file, unlike other compile-time directives (such as
2301L<strict>). Using local() on it would bind its value strictly to a lexical
2302block. Now it is always lexically scoped.
2303
60cf4914 2304As of Perl v5.16.0, it is implemented by the L<arybase> module. See
b82b06b8 2305L<arybase> for more details on its behaviour.
84dabc03 2306
6b54f8ab
FC
2307Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2308effect, and always contains 0. Assigning 0 to it is permitted, but any
2309other value will produce an error.
2310
b82b06b8
FC
2311Mnemonic: [ begins subscripts.
2312
60cf4914 2313Deprecated in Perl v5.12.0.
e1dccc0d 2314
b0c22438 2315=back
2b92dfce 2316
0b9346e6 2317=cut