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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
9The following names have special meaning to Perl. Most of the
5f05dabc 10punctuation names have reasonable mnemonics, or analogues in one of
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11the shells. Nevertheless, if you wish to use the long variable names,
12you just need to say
13
14 use English;
15
16at the top of your program. This will alias all the short names to the
17long names in the current package. Some of them even have medium names,
18generally borrowed from B<awk>.
19
20To go a step further, those variables that depend on the currently
21selected filehandle may instead be set by calling an object method on
22the FileHandle object. (Summary lines below for this contain the word
23HANDLE.) First you must say
24
25 use FileHandle;
26
27after which you may use either
28
29 method HANDLE EXPR
30
31or
32
33 HANDLE->method(EXPR)
34
35Each of the methods returns the old value of the FileHandle attribute.
36The methods each take an optional EXPR, which if supplied specifies the
37new value for the FileHandle attribute in question. If not supplied,
38most of the methods do nothing to the current value, except for
39autoflush(), which will assume a 1 for you, just to be different.
40
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41A few of these variables are considered "read-only". This means that if
42you try to assign to this variable, either directly or indirectly through
43a reference, you'll raise a run-time exception.
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44
45=over 8
46
47=item $ARG
48
49=item $_
50
51The default input and pattern-searching space. The following pairs are
52equivalent:
53
5f05dabc 54 while (<>) {...} # equivalent in only while!
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55 while ($_ = <>) {...}
56
57 /^Subject:/
58 $_ =~ /^Subject:/
59
60 tr/a-z/A-Z/
61 $_ =~ tr/a-z/A-Z/
62
63 chop
64 chop($_)
65
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66Here are the places where Perl will assume $_ even if you
67don't use it:
68
69=over 3
70
71=item *
72
73Various unary functions, including functions like ord() and int(), as well
74as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
75STDIN.
76
77=item *
78
79Various list functions like print() and unlink().
80
81=item *
82
83The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
84without an C<=~> operator.
85
86=item *
87
88The default iterator variable in a C<foreach> loop if no other
89variable is supplied.
90
91=item *
92
93The implicit iterator variable in the grep() and map() functions.
94
95=item *
96
97The default place to put an input record when a C<E<lt>FHE<gt>>
98operation's result is tested by itself as the sole criterion of a C<while>
99test. Note that outside of a C<while> test, this will not happen.
100
101=back
102
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103(Mnemonic: underline is understood in certain operations.)
104
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105=back
106
107=over 8
108
a8f8344d 109=item $E<lt>I<digit>E<gt>
a0d0e21e 110
5f05dabc 111Contains the sub-pattern from the corresponding set of parentheses in
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112the last pattern matched, not counting patterns matched in nested
113blocks that have been exited already. (Mnemonic: like \digit.)
114These variables are all read-only.
115
116=item $MATCH
117
118=item $&
119
120The string matched by the last successful pattern match (not counting
121any matches hidden within a BLOCK or eval() enclosed by the current
122BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
123
124=item $PREMATCH
125
126=item $`
127
128The string preceding whatever was matched by the last successful
129pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 130enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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131string.) This variable is read-only.
132
133=item $POSTMATCH
134
135=item $'
136
137The string following whatever was matched by the last successful
138pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 139enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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140string.) Example:
141
142 $_ = 'abcdefghi';
143 /def/;
144 print "$`:$&:$'\n"; # prints abc:def:ghi
145
146This variable is read-only.
147
148=item $LAST_PAREN_MATCH
149
150=item $+
151
152The last bracket matched by the last search pattern. This is useful if
153you don't know which of a set of alternative patterns matched. For
154example:
155
156 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
157
158(Mnemonic: be positive and forward looking.)
159This variable is read-only.
160
161=item $MULTILINE_MATCHING
162
163=item $*
164
5f05dabc 165Set to 1 to do multi-line matching within a string, 0 to tell Perl
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166that it can assume that strings contain a single line, for the purpose
167of optimizing pattern matches. Pattern matches on strings containing
168multiple newlines can produce confusing results when "C<$*>" is 0. Default
169is 0. (Mnemonic: * matches multiple things.) Note that this variable
5f05dabc 170influences the interpretation of only "C<^>" and "C<$>". A literal newline can
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171be searched for even when C<$* == 0>.
172
5f05dabc 173Use of "C<$*>" is deprecated in modern perls.
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174
175=item input_line_number HANDLE EXPR
176
177=item $INPUT_LINE_NUMBER
178
179=item $NR
180
181=item $.
182
6e2995f4 183The current input line number for the last file handle from
a8f8344d 184which you read (or performed a C<seek> or C<tell> on). An
5f05dabc 185explicit close on a filehandle resets the line number. Because
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186"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
187across ARGV files (but see examples under eof()). Localizing C<$.> has
188the effect of also localizing Perl's notion of "the last read
189filehandle". (Mnemonic: many programs use "." to mean the current line
190number.)
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191
192=item input_record_separator HANDLE EXPR
193
194=item $INPUT_RECORD_SEPARATOR
195
196=item $RS
197
198=item $/
199
200The input record separator, newline by default. Works like B<awk>'s RS
303f2f76 201variable, including treating empty lines as delimiters if set to the
a8f8344d 202null string. (Note: An empty line cannot contain any spaces or
303f2f76 203tabs.) You may set it to a multicharacter string to match a
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204multi-character delimiter. Note that setting it to C<"\n\n"> means
205something slightly different than setting it to C<"">, if the file
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206contains consecutive empty lines. Setting it to C<""> will treat two
207or more consecutive empty lines as a single empty line. Setting it to
208C<"\n\n"> will blindly assume that the next input character belongs to
209the next paragraph, even if it's a newline. (Mnemonic: / is used to
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210delimit line boundaries when quoting poetry.)
211
212 undef $/;
213 $_ = <FH>; # whole file now here
214 s/\n[ \t]+/ /g;
215
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216Remember: the value of $/ is a string, not a regexp. AWK has to be
217better for something :-)
218
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219=item autoflush HANDLE EXPR
220
221=item $OUTPUT_AUTOFLUSH
222
223=item $|
224
225If set to nonzero, forces a flush after every write or print on the
6e2995f4 226currently selected output channel. Default is 0 (regardless of whether
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227the channel is actually buffered by the system or not; C<$|> tells you
228only whether you've asked Perl explicitly to flush after each write).
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229Note that STDOUT will typically be line buffered if output is to the
230terminal and block buffered otherwise. Setting this variable is useful
231primarily when you are outputting to a pipe, such as when you are running
232a Perl script under rsh and want to see the output as it's happening. This
233has no effect on input buffering.
cb1a09d0 234(Mnemonic: when you want your pipes to be piping hot.)
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235
236=item output_field_separator HANDLE EXPR
237
238=item $OUTPUT_FIELD_SEPARATOR
239
240=item $OFS
241
242=item $,
243
244The output field separator for the print operator. Ordinarily the
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245print operator simply prints out the comma-separated fields you
246specify. To get behavior more like B<awk>, set this variable
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247as you would set B<awk>'s OFS variable to specify what is printed
248between fields. (Mnemonic: what is printed when there is a , in your
249print statement.)
250
251=item output_record_separator HANDLE EXPR
252
253=item $OUTPUT_RECORD_SEPARATOR
254
255=item $ORS
256
257=item $\
258
259The output record separator for the print operator. Ordinarily the
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260print operator simply prints out the comma-separated fields you
261specify, with no trailing newline or record separator assumed.
262To get behavior more like B<awk>, set this variable as you would
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263set B<awk>'s ORS variable to specify what is printed at the end of the
264print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
a8f8344d 265print. Also, it's just like C<$/>, but it's what you get "back" from
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266Perl.)
267
268=item $LIST_SEPARATOR
269
270=item $"
271
272This is like "C<$,>" except that it applies to array values interpolated
273into a double-quoted string (or similar interpreted string). Default
274is a space. (Mnemonic: obvious, I think.)
275
276=item $SUBSCRIPT_SEPARATOR
277
278=item $SUBSEP
279
280=item $;
281
282The subscript separator for multi-dimensional array emulation. If you
283refer to a hash element as
284
285 $foo{$a,$b,$c}
286
287it really means
288
289 $foo{join($;, $a, $b, $c)}
290
291But don't put
292
293 @foo{$a,$b,$c} # a slice--note the @
294
295which means
296
297 ($foo{$a},$foo{$b},$foo{$c})
298
299Default is "\034", the same as SUBSEP in B<awk>. Note that if your
300keys contain binary data there might not be any safe value for "C<$;>".
301(Mnemonic: comma (the syntactic subscript separator) is a
302semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
303taken for something more important.)
304
5f05dabc 305Consider using "real" multi-dimensional arrays.
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306
307=item $OFMT
308
309=item $#
310
311The output format for printed numbers. This variable is a half-hearted
312attempt to emulate B<awk>'s OFMT variable. There are times, however,
313when B<awk> and Perl have differing notions of what is in fact
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314numeric. The initial value is %.I<n>g, where I<n> is the value
315of the macro DBL_DIG from your system's F<float.h>. This is different from
316B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
317explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 318
5f05dabc 319Use of "C<$#>" is deprecated.
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320
321=item format_page_number HANDLE EXPR
322
323=item $FORMAT_PAGE_NUMBER
324
325=item $%
326
327The current page number of the currently selected output channel.
328(Mnemonic: % is page number in B<nroff>.)
329
330=item format_lines_per_page HANDLE EXPR
331
332=item $FORMAT_LINES_PER_PAGE
333
334=item $=
335
336The current page length (printable lines) of the currently selected
337output channel. Default is 60. (Mnemonic: = has horizontal lines.)
338
339=item format_lines_left HANDLE EXPR
340
341=item $FORMAT_LINES_LEFT
342
343=item $-
344
345The number of lines left on the page of the currently selected output
346channel. (Mnemonic: lines_on_page - lines_printed.)
347
348=item format_name HANDLE EXPR
349
350=item $FORMAT_NAME
351
352=item $~
353
354The name of the current report format for the currently selected output
355channel. Default is name of the filehandle. (Mnemonic: brother to
356"C<$^>".)
357
358=item format_top_name HANDLE EXPR
359
360=item $FORMAT_TOP_NAME
361
362=item $^
363
364The name of the current top-of-page format for the currently selected
365output channel. Default is name of the filehandle with _TOP
366appended. (Mnemonic: points to top of page.)
367
368=item format_line_break_characters HANDLE EXPR
369
370=item $FORMAT_LINE_BREAK_CHARACTERS
371
372=item $:
373
374The current set of characters after which a string may be broken to
375fill continuation fields (starting with ^) in a format. Default is
376S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
377poetry is a part of a line.)
378
379=item format_formfeed HANDLE EXPR
380
381=item $FORMAT_FORMFEED
382
383=item $^L
384
5f05dabc 385What formats output to perform a form feed. Default is \f.
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386
387=item $ACCUMULATOR
388
389=item $^A
390
391The current value of the write() accumulator for format() lines. A format
392contains formline() commands that put their result into C<$^A>. After
393calling its format, write() prints out the contents of C<$^A> and empties.
394So you never actually see the contents of C<$^A> unless you call
395formline() yourself and then look at it. See L<perlform> and
396L<perlfunc/formline()>.
397
398=item $CHILD_ERROR
399
400=item $?
401
5f05dabc 402The status returned by the last pipe close, back-tick (C<``>) command,
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403or system() operator. Note that this is the status word returned by
404the wait() system call (or else is made up to look like it). Thus,
405the exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
406C<$? & 255> gives which signal, if any, the process died from, and
407whether there was a core dump. (Mnemonic: similar to B<sh> and
408B<ksh>.)
a0d0e21e 409
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410Note that if you have installed a signal handler for C<SIGCHLD>, the
411value of C<$?> will usually be wrong outside that handler.
412
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413Inside an C<END> subroutine C<$?> contains the value that is going to be
414given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
415change the exit status of the script.
416
aa689395 417Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
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418actual VMS exit status, instead of the default emulation of POSIX
419status.
f86702cc 420
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421=item $OS_ERROR
422
423=item $ERRNO
424
425=item $!
426
427If used in a numeric context, yields the current value of errno, with
428all the usual caveats. (This means that you shouldn't depend on the
429value of "C<$!>" to be anything in particular unless you've gotten a
430specific error return indicating a system error.) If used in a string
431context, yields the corresponding system error string. You can assign
5f05dabc 432to "C<$!>" to set I<errno> if, for instance, you want "C<$!>" to return the
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433string for error I<n>, or you want to set the exit value for the die()
434operator. (Mnemonic: What just went bang?)
435
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436=item $EXTENDED_OS_ERROR
437
438=item $^E
439
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440More specific information about the last system error than that provided by
441C<$!>, if available. (If not, it's just C<$!> again, except under OS/2.)
5f05dabc 442At the moment, this differs from C<$!> under only VMS and OS/2, where it
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443provides the VMS status value from the last system error, and OS/2 error
444code of the last call to OS/2 API which was not directed via CRT. The
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445caveats mentioned in the description of C<$!> apply here, too.
446(Mnemonic: Extra error explanation.)
447
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448Note that under OS/2 C<$!> and C<$^E> do not track each other, so if an
449OS/2-specific call is performed, you may need to check both.
5c055ba3 450
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451=item $EVAL_ERROR
452
453=item $@
454
455The Perl syntax error message from the last eval() command. If null, the
456last eval() parsed and executed correctly (although the operations you
457invoked may have failed in the normal fashion). (Mnemonic: Where was
458the syntax error "at"?)
459
748a9306 460Note that warning messages are not collected in this variable. You can,
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461however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
462below.
748a9306 463
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464=item $PROCESS_ID
465
466=item $PID
467
468=item $$
469
470The process number of the Perl running this script. (Mnemonic: same
471as shells.)
472
473=item $REAL_USER_ID
474
475=item $UID
476
477=item $<
478
479The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
480if you're running setuid.)
481
482=item $EFFECTIVE_USER_ID
483
484=item $EUID
485
486=item $>
487
488The effective uid of this process. Example:
489
490 $< = $>; # set real to effective uid
491 ($<,$>) = ($>,$<); # swap real and effective uid
492
493(Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
5f05dabc 494"C<$E<lt>>" and "C<$E<gt>>" can be swapped on only machines supporting setreuid().
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495
496=item $REAL_GROUP_ID
497
498=item $GID
499
500=item $(
501
502The real gid of this process. If you are on a machine that supports
503membership in multiple groups simultaneously, gives a space separated
504list of groups you are in. The first number is the one returned by
505getgid(), and the subsequent ones by getgroups(), one of which may be
506the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
507things. The real gid is the group you I<LEFT>, if you're running setgid.)
508
509=item $EFFECTIVE_GROUP_ID
510
511=item $EGID
512
513=item $)
514
515The effective gid of this process. If you are on a machine that
516supports membership in multiple groups simultaneously, gives a space
517separated list of groups you are in. The first number is the one
518returned by getegid(), and the subsequent ones by getgroups(), one of
519which may be the same as the first number. (Mnemonic: parentheses are
520used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
521you, if you're running setgid.)
522
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523Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
524machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
525and "C<$)>" can be swapped on only machines supporting setregid(). Because
526Perl doesn't currently use initgroups(), you can't set your group vector to
527multiple groups.
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528
529=item $PROGRAM_NAME
530
531=item $0
532
533Contains the name of the file containing the Perl script being
534executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
535program sees. This is more useful as a way of indicating the
536current program state than it is for hiding the program you're running.
537(Mnemonic: same as B<sh> and B<ksh>.)
538
539=item $[
540
541The index of the first element in an array, and of the first character
542in a substring. Default is 0, but you could set it to 1 to make
543Perl behave more like B<awk> (or Fortran) when subscripting and when
544evaluating the index() and substr() functions. (Mnemonic: [ begins
545subscripts.)
546
547As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
548and cannot influence the behavior of any other file. Its use is
549discouraged.
550
551=item $PERL_VERSION
552
553=item $]
554
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555The string printed out when you say C<perl -v>.
556(This is currently I<BROKEN>).
557It can be used to
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558determine at the beginning of a script whether the perl interpreter
559executing the script is in the right range of versions. If used in a
560numeric context, returns the version + patchlevel / 1000. Example:
561
562 # see if getc is available
563 ($version,$patchlevel) =
564 $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
565 print STDERR "(No filename completion available.)\n"
566 if $version * 1000 + $patchlevel < 2016;
567
568or, used numerically,
569
570 warn "No checksumming!\n" if $] < 3.019;
571
572(Mnemonic: Is this version of perl in the right bracket?)
573
574=item $DEBUGGING
575
576=item $^D
577
578The current value of the debugging flags. (Mnemonic: value of B<-D>
579switch.)
580
581=item $SYSTEM_FD_MAX
582
583=item $^F
584
585The maximum system file descriptor, ordinarily 2. System file
586descriptors are passed to exec()ed processes, while higher file
587descriptors are not. Also, during an open(), system file descriptors are
588preserved even if the open() fails. (Ordinary file descriptors are
589closed before the open() is attempted.) Note that the close-on-exec
590status of a file descriptor will be decided according to the value of
591C<$^F> at the time of the open, not the time of the exec.
592
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593=item $^H
594
595The current set of syntax checks enabled by C<use strict>. See the
596documentation of C<strict> for more details.
597
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598=item $INPLACE_EDIT
599
600=item $^I
601
602The current value of the inplace-edit extension. Use C<undef> to disable
603inplace editing. (Mnemonic: value of B<-i> switch.)
604
5c055ba3 605=item $OSNAME
6e2995f4 606
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607=item $^O
608
609The name of the operating system under which this copy of Perl was
610built, as determined during the configuration process. The value
611is identical to C<$Config{'osname'}>.
612
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613=item $PERLDB
614
615=item $^P
616
617The internal flag that the debugger clears so that it doesn't debug
5c055ba3 618itself. You could conceivably disable debugging yourself by clearing
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619it.
620
621=item $BASETIME
622
623=item $^T
624
625The time at which the script began running, in seconds since the
5f05dabc 626epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
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627and B<-C> filetests are
628based on this value.
629
630=item $WARNING
631
632=item $^W
633
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634The current value of the warning switch, either TRUE or FALSE.
635(Mnemonic: related to the B<-w> switch.)
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636
637=item $EXECUTABLE_NAME
638
639=item $^X
640
641The name that the Perl binary itself was executed as, from C's C<argv[0]>.
642
643=item $ARGV
644
a8f8344d 645contains the name of the current file when reading from E<lt>E<gt>.
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646
647=item @ARGV
648
649The array @ARGV contains the command line arguments intended for the
650script. Note that C<$#ARGV> is the generally number of arguments minus
5f05dabc 651one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
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652"C<$0>" for the command name.
653
654=item @INC
655
656The array @INC contains the list of places to look for Perl scripts to
657be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
658initially consists of the arguments to any B<-I> command line switches,
6e2995f4 659followed by the default Perl library, probably F</usr/local/lib/perl>,
cb1a09d0 660followed by ".", to represent the current directory. If you need to
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661modify this at runtime, you should use the C<use lib> pragma
662to get the machine-dependent library properly loaded also:
a0d0e21e 663
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664 use lib '/mypath/libdir/';
665 use SomeMod;
303f2f76 666
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667=item %INC
668
669The hash %INC contains entries for each filename that has
670been included via C<do> or C<require>. The key is the filename you
671specified, and the value is the location of the file actually found.
672The C<require> command uses this array to determine whether a given file
673has already been included.
674
675=item $ENV{expr}
676
677The hash %ENV contains your current environment. Setting a
678value in C<ENV> changes the environment for child processes.
679
680=item $SIG{expr}
681
682The hash %SIG is used to set signal handlers for various
683signals. Example:
684
685 sub handler { # 1st argument is signal name
686 local($sig) = @_;
687 print "Caught a SIG$sig--shutting down\n";
688 close(LOG);
689 exit(0);
690 }
691
692 $SIG{'INT'} = 'handler';
693 $SIG{'QUIT'} = 'handler';
694 ...
695 $SIG{'INT'} = 'DEFAULT'; # restore default action
696 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
697
5f05dabc 698The %SIG array contains values for only the signals actually set within
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699the Perl script. Here are some other examples:
700
701 $SIG{PIPE} = Plumber; # SCARY!!
702 $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
703 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
704 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
705
706The one marked scary is problematic because it's a bareword, which means
707sometimes it's a string representing the function, and sometimes it's
708going to call the subroutine call right then and there! Best to be sure
a8f8344d 709and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
748a9306 710
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711If your system has the sigaction() function then signal handlers are
712installed using it. This means you get reliable signal handling. If
713your system has the SA_RESTART flag it is used when signals handlers are
714installed. This means that system calls for which it is supported
715continue rather than returning when a signal arrives. If you want your
716system calls to be interrupted by signal delivery then do something like
717this:
718
719 use POSIX ':signal_h';
720
721 my $alarm = 0;
722 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
723 or die "Error setting SIGALRM handler: $!\n";
724
725See L<POSIX>.
726
748a9306 727Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 728routine indicated by C<$SIG{__WARN__}> is called when a warning message is
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729about to be printed. The warning message is passed as the first
730argument. The presence of a __WARN__ hook causes the ordinary printing
731of warnings to STDERR to be suppressed. You can use this to save warnings
732in a variable, or turn warnings into fatal errors, like this:
733
734 local $SIG{__WARN__} = sub { die $_[0] };
735 eval $proggie;
736
a8f8344d 737The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
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738is about to be thrown. The error message is passed as the first
739argument. When a __DIE__ hook routine returns, the exception
740processing continues as it would have in the absence of the hook,
cb1a09d0 741unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
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742The C<__DIE__> handler is explicitly disabled during the call, so that you
743can die from a C<__DIE__> handler. Similarly for C<__WARN__>. See
744L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval>.
a0d0e21e 745
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746=item $^M
747
748By default, running out of memory it is not trappable. However, if
749compiled for this, Perl may use the contents of C<$^M> as an emergency
750pool after die()ing with this message. Suppose that your Perl were
751compiled with -DEMERGENCY_SBRK and used Perl's malloc. Then
752
753 $^M = 'a' x (1<<16);
754
755would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
756file for information on how to enable this option. As a disincentive to
757casual use of this advanced feature, there is no L<English> long name for
758this variable.
759
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