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patch [bleadperl]: Document %! special variable
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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
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10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
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13
14 use English;
15
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16at the top of your program. This aliases all the short names to the long
17names in the current package. Some even have medium names, generally
18borrowed from B<awk>. In general, it's best to use the
a0d0e21e 19
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20 use English '-no_match_vars';
21
22invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23a certain performance hit with the use of regular expressions. See
24L<English>.
25
26Variables that depend on the currently selected filehandle may be set by
27calling an appropriate object method on the IO::Handle object, although
28this is less efficient than using the regular built-in variables. (Summary
29lines below for this contain the word HANDLE.) First you must say
a0d0e21e 30
19799a22 31 use IO::Handle;
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32
33after which you may use either
34
35 method HANDLE EXPR
36
5a964f20 37or more safely,
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38
39 HANDLE->method(EXPR)
40
14218588 41Each method returns the old value of the IO::Handle attribute.
a1ce9542 42The methods each take an optional EXPR, which, if supplied, specifies the
19799a22 43new value for the IO::Handle attribute in question. If not supplied,
14218588 44most methods do nothing to the current value--except for
a0d0e21e 45autoflush(), which will assume a 1 for you, just to be different.
a1ce9542 46
14218588 47Because loading in the IO::Handle class is an expensive operation, you should
19799a22 48learn how to use the regular built-in variables.
a0d0e21e 49
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50A few of these variables are considered "read-only". This means that if
51you try to assign to this variable, either directly or indirectly through
52a reference, you'll raise a run-time exception.
a0d0e21e 53
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54You should be very careful when modifying the default values of most
55special variables described in this document. In most cases you want
56to localize these variables before changing them, since if you don't,
57the change may affect other modules which rely on the default values
58of the special variables that you have changed. This is one of the
59correct ways to read the whole file at once:
60
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
63 my $content = <$fh>;
64 close $fh;
65
66But the following code is quite bad:
67
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
70 my $content = <$fh>;
71 close $fh;
72
73since some other module, may want to read data from some file in the
74default "line mode", so if the code we have just presented has been
75executed, the global value of C<$/> is now changed for any other code
76running inside the same Perl interpreter.
77
78Usually when a variable is localized you want to make sure that this
79change affects the shortest scope possible. So unless you are already
80inside some short C<{}> block, you should create one yourself. For
81example:
82
83 my $content = '';
84 open my $fh, "foo" or die $!;
85 {
86 local $/;
87 $content = <$fh>;
88 }
89 close $fh;
90
91Here is an example of how your own code can go broken:
92
93 for (1..5){
94 nasty_break();
95 print "$_ ";
96 }
97 sub nasty_break {
98 $_ = 5;
99 # do something with $_
100 }
101
102You probably expect this code to print:
103
104 1 2 3 4 5
105
106but instead you get:
107
108 5 5 5 5 5
109
110Why? Because nasty_break() modifies C<$_> without localizing it
111first. The fix is to add local():
112
113 local $_ = 5;
114
115It's easy to notice the problem in such a short example, but in more
116complicated code you are looking for trouble if you don't localize
117changes to the special variables.
118
fb73857a 119The following list is ordered by scalar variables first, then the
87275199 120arrays, then the hashes.
fb73857a 121
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122=over 8
123
124=item $ARG
125
126=item $_
127
128The default input and pattern-searching space. The following pairs are
129equivalent:
130
19799a22 131 while (<>) {...} # equivalent only in while!
54310121 132 while (defined($_ = <>)) {...}
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133
134 /^Subject:/
135 $_ =~ /^Subject:/
136
137 tr/a-z/A-Z/
138 $_ =~ tr/a-z/A-Z/
139
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140 chomp
141 chomp($_)
a0d0e21e 142
54310121 143Here are the places where Perl will assume $_ even if you
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144don't use it:
145
146=over 3
147
148=item *
149
150Various unary functions, including functions like ord() and int(), as well
151as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
152STDIN.
153
154=item *
155
156Various list functions like print() and unlink().
157
158=item *
159
160The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
161without an C<=~> operator.
162
54310121 163=item *
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164
165The default iterator variable in a C<foreach> loop if no other
166variable is supplied.
167
54310121 168=item *
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169
170The implicit iterator variable in the grep() and map() functions.
171
54310121 172=item *
cb1a09d0 173
c47ff5f1 174The default place to put an input record when a C<< <FH> >>
cb1a09d0 175operation's result is tested by itself as the sole criterion of a C<while>
14218588 176test. Outside a C<while> test, this will not happen.
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177
178=back
179
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180(Mnemonic: underline is understood in certain operations.)
181
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182=back
183
184=over 8
185
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186=item $a
187
188=item $b
189
190Special package variables when using sort(), see L<perlfunc/sort>.
191Because of this specialness $a and $b don't need to be declared
192(using local(), use vars, or our()) even when using the strict
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193vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
194if you want to be able to use them in the sort() comparison block
195or function.
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196
197=back
198
199=over 8
200
c47ff5f1 201=item $<I<digits>>
a0d0e21e 202
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203Contains the subpattern from the corresponding set of capturing
204parentheses from the last pattern match, not counting patterns
205matched in nested blocks that have been exited already. (Mnemonic:
206like \digits.) These variables are all read-only and dynamically
207scoped to the current BLOCK.
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208
209=item $MATCH
210
211=item $&
212
213The string matched by the last successful pattern match (not counting
214any matches hidden within a BLOCK or eval() enclosed by the current
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215BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
216and dynamically scoped to the current BLOCK.
a0d0e21e 217
19ddd453 218The use of this variable anywhere in a program imposes a considerable
667e1aea 219performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 220
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221=item $PREMATCH
222
223=item $`
224
225The string preceding whatever was matched by the last successful
226pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 227enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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228string.) This variable is read-only.
229
19ddd453 230The use of this variable anywhere in a program imposes a considerable
667e1aea 231performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 232
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233=item $POSTMATCH
234
235=item $'
236
237The string following whatever was matched by the last successful
238pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 239enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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240string.) Example:
241
22d0716c 242 local $_ = 'abcdefghi';
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243 /def/;
244 print "$`:$&:$'\n"; # prints abc:def:ghi
245
19799a22 246This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 247
19ddd453 248The use of this variable anywhere in a program imposes a considerable
667e1aea 249performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 250
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251=item $LAST_PAREN_MATCH
252
253=item $+
254
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255The text matched by the last bracket of the last successful search pattern.
256This is useful if you don't know which one of a set of alternative patterns
257matched. For example:
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258
259 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
260
261(Mnemonic: be positive and forward looking.)
19799a22 262This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 263
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264=item $^N
265
266The text matched by the used group most-recently closed (i.e. the group
267with the rightmost closing parenthesis) of the last successful search
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268pattern. (Mnemonic: the (possibly) Nested parenthesis that most
269recently closed.)
270
210b36aa 271This is primarily used inside C<(?{...})> blocks for examining text
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272recently matched. For example, to effectively capture text to a variable
273(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
274
275 (?:(...)(?{ $var = $^N }))
276
277By setting and then using C<$var> in this way relieves you from having to
278worry about exactly which numbered set of parentheses they are.
279
280This variable is dynamically scoped to the current BLOCK.
281
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282=item @LAST_MATCH_END
283
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284=item @+
285
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286This array holds the offsets of the ends of the last successful
287submatches in the currently active dynamic scope. C<$+[0]> is
288the offset into the string of the end of the entire match. This
289is the same value as what the C<pos> function returns when called
290on the variable that was matched against. The I<n>th element
291of this array holds the offset of the I<n>th submatch, so
292C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
293past where $2 ends, and so on. You can use C<$#+> to determine
294how many subgroups were in the last successful match. See the
295examples given for the C<@-> variable.
6cef1e77 296
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297=item $MULTILINE_MATCHING
298
299=item $*
300
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301Set to a non-zero integer value to do multi-line matching within a
302string, 0 (or undefined) to tell Perl that it can assume that strings
303contain a single line, for the purpose of optimizing pattern matches.
304Pattern matches on strings containing multiple newlines can produce
305confusing results when C<$*> is 0 or undefined. Default is undefined.
306(Mnemonic: * matches multiple things.) This variable influences the
307interpretation of only C<^> and C<$>. A literal newline can be searched
308for even when C<$* == 0>.
a0d0e21e 309
19799a22 310Use of C<$*> is deprecated in modern Perl, supplanted by
5a964f20 311the C</s> and C</m> modifiers on pattern matching.
a0d0e21e 312
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313Assigning a non-numerical value to C<$*> triggers a warning (and makes
314C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
315makes that an implicit C<int> is applied on the value.
316
fcc7d916 317=item HANDLE->input_line_number(EXPR)
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318
319=item $INPUT_LINE_NUMBER
320
321=item $NR
322
323=item $.
324
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325Current line number for the last filehandle accessed.
326
327Each filehandle in Perl counts the number of lines that have been read
328from it. (Depending on the value of C<$/>, Perl's idea of what
329constitutes a line may not match yours.) When a line is read from a
330filehandle (via readline() or C<< <> >>), or when tell() or seek() is
331called on it, C<$.> becomes an alias to the line counter for that
332filehandle.
333
334You can adjust the counter by assigning to C<$.>, but this will not
335actually move the seek pointer. I<Localizing C<$.> will not localize
336the filehandle's line count>. Instead, it will localize perl's notion
337of which filehandle C<$.> is currently aliased to.
338
339C<$.> is reset when the filehandle is closed, but B<not> when an open
340filehandle is reopened without an intervening close(). For more
e48df184 341details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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342an explicit close, line numbers increase across ARGV files (but see
343examples in L<perlfunc/eof>).
344
345You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
346line counter for a given filehandle without having to worry about
347which handle you last accessed.
348
349(Mnemonic: many programs use "." to mean the current line number.)
350
351=item IO::Handle->input_record_separator(EXPR)
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352
353=item $INPUT_RECORD_SEPARATOR
354
355=item $RS
356
357=item $/
358
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359The input record separator, newline by default. This
360influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 361variable, including treating empty lines as a terminator if set to
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362the null string. (An empty line cannot contain any spaces
363or tabs.) You may set it to a multi-character string to match a
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364multi-character terminator, or to C<undef> to read through the end
365of file. Setting it to C<"\n\n"> means something slightly
366different than setting to C<"">, if the file contains consecutive
367empty lines. Setting to C<""> will treat two or more consecutive
368empty lines as a single empty line. Setting to C<"\n\n"> will
369blindly assume that the next input character belongs to the next
14218588 370paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 371line boundaries when quoting poetry.)
a0d0e21e 372
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373 local $/; # enable "slurp" mode
374 local $_ = <FH>; # whole file now here
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375 s/\n[ \t]+/ /g;
376
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377Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
378better for something. :-)
68dc0745 379
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380Setting C<$/> to a reference to an integer, scalar containing an integer, or
381scalar that's convertible to an integer will attempt to read records
5b2b9c68 382instead of lines, with the maximum record size being the referenced
19799a22 383integer. So this:
5b2b9c68 384
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385 local $/ = \32768; # or \"32768", or \$var_containing_32768
386 open my $fh, $myfile or die $!;
387 local $_ = <$fh>;
5b2b9c68 388
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389will read a record of no more than 32768 bytes from FILE. If you're
390not reading from a record-oriented file (or your OS doesn't have
391record-oriented files), then you'll likely get a full chunk of data
392with every read. If a record is larger than the record size you've
393set, you'll get the record back in pieces.
5b2b9c68 394
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395On VMS, record reads are done with the equivalent of C<sysread>,
396so it's best not to mix record and non-record reads on the same
397file. (This is unlikely to be a problem, because any file you'd
83763826 398want to read in record mode is probably unusable in line mode.)
14218588 399Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 400non-record reads of a file.
5b2b9c68 401
14218588 402See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 403
fcc7d916 404=item HANDLE->autoflush(EXPR)
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405
406=item $OUTPUT_AUTOFLUSH
407
408=item $|
409
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410If set to nonzero, forces a flush right away and after every write
411or print on the currently selected output channel. Default is 0
14218588 412(regardless of whether the channel is really buffered by the
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413system or not; C<$|> tells you only whether you've asked Perl
414explicitly to flush after each write). STDOUT will
415typically be line buffered if output is to the terminal and block
416buffered otherwise. Setting this variable is useful primarily when
417you are outputting to a pipe or socket, such as when you are running
418a Perl program under B<rsh> and want to see the output as it's
419happening. This has no effect on input buffering. See L<perlfunc/getc>
420for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 421
46550894 422=item IO::Handle->output_field_separator EXPR
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423
424=item $OUTPUT_FIELD_SEPARATOR
425
426=item $OFS
427
428=item $,
429
430The output field separator for the print operator. Ordinarily the
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431print operator simply prints out its arguments without further
432adornment. To get behavior more like B<awk>, set this variable as
433you would set B<awk>'s OFS variable to specify what is printed
434between fields. (Mnemonic: what is printed when there is a "," in
435your print statement.)
a0d0e21e 436
46550894 437=item IO::Handle->output_record_separator EXPR
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438
439=item $OUTPUT_RECORD_SEPARATOR
440
441=item $ORS
442
443=item $\
444
445The output record separator for the print operator. Ordinarily the
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446print operator simply prints out its arguments as is, with no
447trailing newline or other end-of-record string added. To get
448behavior more like B<awk>, set this variable as you would set
449B<awk>'s ORS variable to specify what is printed at the end of the
450print. (Mnemonic: you set C<$\> instead of adding "\n" at the
451end of the print. Also, it's just like C<$/>, but it's what you
452get "back" from Perl.)
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453
454=item $LIST_SEPARATOR
455
456=item $"
457
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458This is like C<$,> except that it applies to array and slice values
459interpolated into a double-quoted string (or similar interpreted
460string). Default is a space. (Mnemonic: obvious, I think.)
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461
462=item $SUBSCRIPT_SEPARATOR
463
464=item $SUBSEP
465
466=item $;
467
54310121 468The subscript separator for multidimensional array emulation. If you
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469refer to a hash element as
470
471 $foo{$a,$b,$c}
472
473it really means
474
475 $foo{join($;, $a, $b, $c)}
476
477But don't put
478
479 @foo{$a,$b,$c} # a slice--note the @
480
481which means
482
483 ($foo{$a},$foo{$b},$foo{$c})
484
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485Default is "\034", the same as SUBSEP in B<awk>. If your
486keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 487(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 488semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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489taken for something more important.)
490
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491Consider using "real" multidimensional arrays as described
492in L<perllol>.
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493
494=item $OFMT
495
496=item $#
497
498The output format for printed numbers. This variable is a half-hearted
499attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 500when B<awk> and Perl have differing notions of what counts as
19799a22 501numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 502of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 503B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 504explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 505
19799a22 506Use of C<$#> is deprecated.
a0d0e21e 507
fcc7d916 508=item HANDLE->format_page_number(EXPR)
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509
510=item $FORMAT_PAGE_NUMBER
511
512=item $%
513
514The current page number of the currently selected output channel.
19799a22 515Used with formats.
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516(Mnemonic: % is page number in B<nroff>.)
517
fcc7d916 518=item HANDLE->format_lines_per_page(EXPR)
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519
520=item $FORMAT_LINES_PER_PAGE
521
522=item $=
523
524The current page length (printable lines) of the currently selected
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525output channel. Default is 60.
526Used with formats.
527(Mnemonic: = has horizontal lines.)
a0d0e21e 528
fcc7d916 529=item HANDLE->format_lines_left(EXPR)
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530
531=item $FORMAT_LINES_LEFT
532
533=item $-
534
535The number of lines left on the page of the currently selected output
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536channel.
537Used with formats.
538(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 539
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540=item @LAST_MATCH_START
541
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542=item @-
543
19799a22 544$-[0] is the offset of the start of the last successful match.
6cef1e77 545C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 546I<n>-th subpattern, or undef if the subpattern did not match.
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547
548Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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549$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
550$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
c47ff5f1 551C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
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552matched subgroup in the last successful match. Contrast with
553C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 554with C<@+>.
6cef1e77 555
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556This array holds the offsets of the beginnings of the last
557successful submatches in the currently active dynamic scope.
558C<$-[0]> is the offset into the string of the beginning of the
559entire match. The I<n>th element of this array holds the offset
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560of the I<n>th submatch, so C<$-[1]> is the offset where $1
561begins, C<$-[2]> the offset where $2 begins, and so on.
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562
563After a match against some variable $var:
564
565=over 5
566
4375e838 567=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 568
4375e838 569=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 570
4375e838 571=item C<$'> is the same as C<substr($var, $+[0])>
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572
573=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
574
575=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
576
4375e838 577=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
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578
579=back
580
fcc7d916 581=item HANDLE->format_name(EXPR)
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582
583=item $FORMAT_NAME
584
585=item $~
586
587The name of the current report format for the currently selected output
14218588 588channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 589C<$^>.)
a0d0e21e 590
fcc7d916 591=item HANDLE->format_top_name(EXPR)
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592
593=item $FORMAT_TOP_NAME
594
595=item $^
596
597The name of the current top-of-page format for the currently selected
14218588 598output channel. Default is the name of the filehandle with _TOP
a0d0e21e
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599appended. (Mnemonic: points to top of page.)
600
46550894 601=item IO::Handle->format_line_break_characters EXPR
a0d0e21e
LW
602
603=item $FORMAT_LINE_BREAK_CHARACTERS
604
605=item $:
606
607The current set of characters after which a string may be broken to
54310121 608fill continuation fields (starting with ^) in a format. Default is
a0d0e21e
LW
609S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
610poetry is a part of a line.)
611
46550894 612=item IO::Handle->format_formfeed EXPR
a0d0e21e
LW
613
614=item $FORMAT_FORMFEED
615
616=item $^L
617
14218588 618What formats output as a form feed. Default is \f.
a0d0e21e
LW
619
620=item $ACCUMULATOR
621
622=item $^A
623
624The current value of the write() accumulator for format() lines. A format
19799a22 625contains formline() calls that put their result into C<$^A>. After
a0d0e21e 626calling its format, write() prints out the contents of C<$^A> and empties.
14218588 627So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
628formline() yourself and then look at it. See L<perlform> and
629L<perlfunc/formline()>.
630
631=item $CHILD_ERROR
632
633=item $?
634
54310121 635The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
636successful call to wait() or waitpid(), or from the system()
637operator. This is just the 16-bit status word returned by the
638wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 639exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
640C<$? & 127> gives which signal, if any, the process died from, and
641C<$? & 128> reports whether there was a core dump. (Mnemonic:
642similar to B<sh> and B<ksh>.)
a0d0e21e 643
7b8d334a 644Additionally, if the C<h_errno> variable is supported in C, its value
14218588 645is returned via $? if any C<gethost*()> function fails.
7b8d334a 646
19799a22 647If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
648value of C<$?> will usually be wrong outside that handler.
649
a8f8344d
PP
650Inside an C<END> subroutine C<$?> contains the value that is going to be
651given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
652change the exit status of your program. For example:
653
654 END {
655 $? = 1 if $? == 255; # die would make it 255
656 }
a8f8344d 657
aa689395 658Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 659actual VMS exit status, instead of the default emulation of POSIX
9bc98430 660status; see L<perlvms/$?> for details.
f86702cc 661
55602bd2
IZ
662Also see L<Error Indicators>.
663
0a378802
JH
664=item ${^ENCODING}
665
666The encoding used to interpret native eight-bit encodings to Unicode,
667see L<encode>. An opaque C<Encode::XS> object.
668
a0d0e21e
LW
669=item $OS_ERROR
670
671=item $ERRNO
672
673=item $!
674
19799a22 675If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
676variable, or in other words, if a system or library call fails, it
677sets this variable. This means that the value of C<$!> is meaningful
678only I<immediately> after a B<failure>:
679
680 if (open(FH, $filename)) {
681 # Here $! is meaningless.
682 ...
683 } else {
684 # ONLY here is $! meaningful.
685 ...
686 # Already here $! might be meaningless.
687 }
688 # Since here we might have either success or failure,
689 # here $! is meaningless.
690
691In the above I<meaningless> stands for anything: zero, non-zero,
692C<undef>. A successful system or library call does B<not> set
693the variable to zero.
694
19799a22
GS
695If used an a string, yields the corresponding system error string.
696You can assign a number to C<$!> to set I<errno> if, for instance,
697you want C<"$!"> to return the string for error I<n>, or you want
698to set the exit value for the die() operator. (Mnemonic: What just
699went bang?)
a0d0e21e 700
55602bd2
IZ
701Also see L<Error Indicators>.
702
4c5cef9b
MJD
703=item %!
704
705Each element of C<%!> has a true value only if C<$!> is set to that
706value. For example, C<$!{ENOENT}> is true if and only if the current
707value of C<$!> is C<ENOENT>; that is, if the mot recent error was "No
708such file or directory". To check if a particular key is meaningful
709on your system, use C<exists $!{the_key}>; for a list of legal keys,
710use C<keys %!>. See L<Errno> for more information.
711
5c055ba3
PP
712=item $EXTENDED_OS_ERROR
713
714=item $^E
715
22fae026
TM
716Error information specific to the current operating system. At
717the moment, this differs from C<$!> under only VMS, OS/2, and Win32
718(and for MacPerl). On all other platforms, C<$^E> is always just
719the same as C<$!>.
720
721Under VMS, C<$^E> provides the VMS status value from the last
722system error. This is more specific information about the last
723system error than that provided by C<$!>. This is particularly
d516a115 724important when C<$!> is set to B<EVMSERR>.
22fae026 725
1c1c7f20
GS
726Under OS/2, C<$^E> is set to the error code of the last call to
727OS/2 API either via CRT, or directly from perl.
22fae026
TM
728
729Under Win32, C<$^E> always returns the last error information
730reported by the Win32 call C<GetLastError()> which describes
731the last error from within the Win32 API. Most Win32-specific
19799a22 732code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
733set C<errno> and so most portable Perl code will report errors
734via C<$!>.
735
736Caveats mentioned in the description of C<$!> generally apply to
737C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 738
55602bd2
IZ
739Also see L<Error Indicators>.
740
a0d0e21e
LW
741=item $EVAL_ERROR
742
743=item $@
744
4a280ebe
JG
745The Perl syntax error message from the last eval() operator.
746If $@ is the null string, the last eval() parsed and executed
747correctly (although the operations you invoked may have failed in the
748normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 749
19799a22 750Warning messages are not collected in this variable. You can,
a8f8344d 751however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 752as described below.
748a9306 753
55602bd2
IZ
754Also see L<Error Indicators>.
755
a0d0e21e
LW
756=item $PROCESS_ID
757
758=item $PID
759
760=item $$
761
19799a22
GS
762The process number of the Perl running this script. You should
763consider this variable read-only, although it will be altered
764across fork() calls. (Mnemonic: same as shells.)
a0d0e21e
LW
765
766=item $REAL_USER_ID
767
768=item $UID
769
770=item $<
771
19799a22 772The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685
GW
773if you're running setuid.) You can change both the real uid and
774the effective uid at the same time by using POSIX::setuid().
a0d0e21e
LW
775
776=item $EFFECTIVE_USER_ID
777
778=item $EUID
779
780=item $>
781
782The effective uid of this process. Example:
783
784 $< = $>; # set real to effective uid
785 ($<,$>) = ($>,$<); # swap real and effective uid
786
a043a685
GW
787You can change both the effective uid and the real uid at the same
788time by using POSIX::setuid().
789
19799a22 790(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 791C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 792supporting setreuid().
a0d0e21e
LW
793
794=item $REAL_GROUP_ID
795
796=item $GID
797
798=item $(
799
800The real gid of this process. If you are on a machine that supports
801membership in multiple groups simultaneously, gives a space separated
802list of groups you are in. The first number is the one returned by
803getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
804the same as the first number.
805
19799a22
GS
806However, a value assigned to C<$(> must be a single number used to
807set the real gid. So the value given by C<$(> should I<not> be assigned
808back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 809
a043a685
GW
810You can change both the real gid and the effective gid at the same
811time by using POSIX::setgid().
812
19799a22
GS
813(Mnemonic: parentheses are used to I<group> things. The real gid is the
814group you I<left>, if you're running setgid.)
a0d0e21e
LW
815
816=item $EFFECTIVE_GROUP_ID
817
818=item $EGID
819
820=item $)
821
822The effective gid of this process. If you are on a machine that
823supports membership in multiple groups simultaneously, gives a space
824separated list of groups you are in. The first number is the one
825returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
826which may be the same as the first number.
827
19799a22 828Similarly, a value assigned to C<$)> must also be a space-separated
14218588 829list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
830the rest (if any) are passed to setgroups(). To get the effect of an
831empty list for setgroups(), just repeat the new effective gid; that is,
832to force an effective gid of 5 and an effectively empty setgroups()
833list, say C< $) = "5 5" >.
834
a043a685
GW
835You can change both the effective gid and the real gid at the same
836time by using POSIX::setgid() (use only a single numeric argument).
837
19799a22
GS
838(Mnemonic: parentheses are used to I<group> things. The effective gid
839is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 840
c47ff5f1 841C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
842machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
843and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
844
845=item $PROGRAM_NAME
846
847=item $0
848
19799a22
GS
849Contains the name of the program being executed. On some operating
850systems assigning to C<$0> modifies the argument area that the B<ps>
851program sees. This is more useful as a way of indicating the current
852program state than it is for hiding the program you're running.
a0d0e21e
LW
853(Mnemonic: same as B<sh> and B<ksh>.)
854
4bc88a62
PS
855Note for BSD users: setting C<$0> does not completely remove "perl"
856from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
857result in C<"perl: foobar (perl)">. This is an operating system
858feature.
859
a0d0e21e
LW
860=item $[
861
862The index of the first element in an array, and of the first character
19799a22
GS
863in a substring. Default is 0, but you could theoretically set it
864to 1 to make Perl behave more like B<awk> (or Fortran) when
865subscripting and when evaluating the index() and substr() functions.
866(Mnemonic: [ begins subscripts.)
a0d0e21e 867
19799a22
GS
868As of release 5 of Perl, assignment to C<$[> is treated as a compiler
869directive, and cannot influence the behavior of any other file.
870Its use is highly discouraged.
a0d0e21e 871
a0d0e21e
LW
872=item $]
873
54310121
PP
874The version + patchlevel / 1000 of the Perl interpreter. This variable
875can be used to determine whether the Perl interpreter executing a
876script is in the right range of versions. (Mnemonic: Is this version
877of perl in the right bracket?) Example:
a0d0e21e
LW
878
879 warn "No checksumming!\n" if $] < 3.019;
880
54310121 881See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 882for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 883
0c8d858b
MS
884The floating point representation can sometimes lead to inaccurate
885numeric comparisons. See C<$^V> for a more modern representation of
886the Perl version that allows accurate string comparisons.
16070b82 887
305aace0
NIS
888=item $COMPILING
889
890=item $^C
891
19799a22
GS
892The current value of the flag associated with the B<-c> switch.
893Mainly of use with B<-MO=...> to allow code to alter its behavior
894when being compiled, such as for example to AUTOLOAD at compile
895time rather than normal, deferred loading. See L<perlcc>. Setting
896C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 897
a0d0e21e
LW
898=item $DEBUGGING
899
900=item $^D
901
902The current value of the debugging flags. (Mnemonic: value of B<-D>
903switch.)
904
905=item $SYSTEM_FD_MAX
906
907=item $^F
908
909The maximum system file descriptor, ordinarily 2. System file
910descriptors are passed to exec()ed processes, while higher file
911descriptors are not. Also, during an open(), system file descriptors are
912preserved even if the open() fails. (Ordinary file descriptors are
19799a22 913closed before the open() is attempted.) The close-on-exec
a0d0e21e 914status of a file descriptor will be decided according to the value of
8d2a6795
GS
915C<$^F> when the corresponding file, pipe, or socket was opened, not the
916time of the exec().
a0d0e21e 917
6e2995f4
PP
918=item $^H
919
0462a1ab
GS
920WARNING: This variable is strictly for internal use only. Its availability,
921behavior, and contents are subject to change without notice.
922
923This variable contains compile-time hints for the Perl interpreter. At the
924end of compilation of a BLOCK the value of this variable is restored to the
925value when the interpreter started to compile the BLOCK.
926
927When perl begins to parse any block construct that provides a lexical scope
928(e.g., eval body, required file, subroutine body, loop body, or conditional
929block), the existing value of $^H is saved, but its value is left unchanged.
930When the compilation of the block is completed, it regains the saved value.
931Between the points where its value is saved and restored, code that
932executes within BEGIN blocks is free to change the value of $^H.
933
934This behavior provides the semantic of lexical scoping, and is used in,
935for instance, the C<use strict> pragma.
936
937The contents should be an integer; different bits of it are used for
938different pragmatic flags. Here's an example:
939
940 sub add_100 { $^H |= 0x100 }
941
942 sub foo {
943 BEGIN { add_100() }
944 bar->baz($boon);
945 }
946
947Consider what happens during execution of the BEGIN block. At this point
948the BEGIN block has already been compiled, but the body of foo() is still
949being compiled. The new value of $^H will therefore be visible only while
950the body of foo() is being compiled.
951
952Substitution of the above BEGIN block with:
953
954 BEGIN { require strict; strict->import('vars') }
955
956demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
957version of the same lexical pragma:
958
959 BEGIN { require strict; strict->import('vars') if $condition }
960
961=item %^H
962
963WARNING: This variable is strictly for internal use only. Its availability,
964behavior, and contents are subject to change without notice.
965
966The %^H hash provides the same scoping semantic as $^H. This makes it
967useful for implementation of lexically scoped pragmas.
6e2995f4 968
a0d0e21e
LW
969=item $INPLACE_EDIT
970
971=item $^I
972
973The current value of the inplace-edit extension. Use C<undef> to disable
974inplace editing. (Mnemonic: value of B<-i> switch.)
975
fb73857a
PP
976=item $^M
977
19799a22
GS
978By default, running out of memory is an untrappable, fatal error.
979However, if suitably built, Perl can use the contents of C<$^M>
980as an emergency memory pool after die()ing. Suppose that your Perl
981were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
982Then
fb73857a 983
19799a22 984 $^M = 'a' x (1 << 16);
fb73857a 985
51ee6500 986would allocate a 64K buffer for use in an emergency. See the
19799a22
GS
987F<INSTALL> file in the Perl distribution for information on how to
988enable this option. To discourage casual use of this advanced
4ec0190b 989feature, there is no L<English|English> long name for this variable.
fb73857a 990
5c055ba3 991=item $OSNAME
6e2995f4 992
5c055ba3
PP
993=item $^O
994
995The name of the operating system under which this copy of Perl was
996built, as determined during the configuration process. The value
19799a22
GS
997is identical to C<$Config{'osname'}>. See also L<Config> and the
998B<-V> command-line switch documented in L<perlrun>.
5c055ba3 999
e2e27056
JH
1000=item ${^OPEN}
1001
1002An internal variable used by PerlIO. A string in two parts, separated
1003by a C<\0> byte, the first part is the input disciplines, the second
1004part is the output disciplines.
1005
a0d0e21e
LW
1006=item $PERLDB
1007
1008=item $^P
1009
19799a22
GS
1010The internal variable for debugging support. The meanings of the
1011various bits are subject to change, but currently indicate:
84902520
TB
1012
1013=over 6
1014
1015=item 0x01
1016
1017Debug subroutine enter/exit.
1018
1019=item 0x02
1020
1021Line-by-line debugging.
1022
1023=item 0x04
1024
1025Switch off optimizations.
1026
1027=item 0x08
1028
1029Preserve more data for future interactive inspections.
1030
1031=item 0x10
1032
1033Keep info about source lines on which a subroutine is defined.
1034
1035=item 0x20
1036
1037Start with single-step on.
1038
83ee9e09
GS
1039=item 0x40
1040
1041Use subroutine address instead of name when reporting.
1042
1043=item 0x80
1044
1045Report C<goto &subroutine> as well.
1046
1047=item 0x100
1048
1049Provide informative "file" names for evals based on the place they were compiled.
1050
1051=item 0x200
1052
1053Provide informative names to anonymous subroutines based on the place they
1054were compiled.
1055
84902520
TB
1056=back
1057
19799a22
GS
1058Some bits may be relevant at compile-time only, some at
1059run-time only. This is a new mechanism and the details may change.
a0d0e21e 1060
66558a10
GS
1061=item $LAST_REGEXP_CODE_RESULT
1062
b9ac3b5b
GS
1063=item $^R
1064
19799a22
GS
1065The result of evaluation of the last successful C<(?{ code })>
1066regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1067
66558a10
GS
1068=item $EXCEPTIONS_BEING_CAUGHT
1069
fb73857a
PP
1070=item $^S
1071
1072Current state of the interpreter. Undefined if parsing of the current
1073module/eval is not finished (may happen in $SIG{__DIE__} and
19799a22 1074$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
fb73857a 1075
a0d0e21e
LW
1076=item $BASETIME
1077
1078=item $^T
1079
19799a22 1080The time at which the program began running, in seconds since the
5f05dabc 1081epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1082and B<-C> filetests are based on this value.
a0d0e21e 1083
7c36658b
MS
1084=item ${^TAINT}
1085
c212f17f 1086Reflects if taint mode is on or off (i.e. if the program was run with
7c36658b
MS
1087B<-T> or not). True for on, false for off.
1088
44dcb63b 1089=item $PERL_VERSION
b459063d 1090
16070b82
GS
1091=item $^V
1092
1093The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1094as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1095it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1096C<$^V eq v5.6.0>. Note that the characters in this string value can
1097potentially be in Unicode range.
16070b82
GS
1098
1099This can be used to determine whether the Perl interpreter executing a
1100script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1101Control.) Example:
16070b82 1102
3fd4402b 1103 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1104
aa2f2a36
AMS
1105To convert C<$^V> into its string representation use sprintf()'s
1106C<"%vd"> conversion:
1107
1108 printf "version is v%vd\n", $^V; # Perl's version
1109
44dcb63b 1110See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1111for a convenient way to fail if the running Perl interpreter is too old.
1112
1113See also C<$]> for an older representation of the Perl version.
1114
a0d0e21e
LW
1115=item $WARNING
1116
1117=item $^W
1118
19799a22
GS
1119The current value of the warning switch, initially true if B<-w>
1120was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1121related to the B<-w> switch.) See also L<warnings>.
1122
6a818117 1123=item ${^WARNING_BITS}
4438c4b7
JH
1124
1125The current set of warning checks enabled by the C<use warnings> pragma.
1126See the documentation of C<warnings> for more details.
a0d0e21e 1127
46487f74
GS
1128=item ${^WIDE_SYSTEM_CALLS}
1129
1130Global flag that enables system calls made by Perl to use wide character
1131APIs native to the system, if available. This is currently only implemented
1132on the Windows platform.
1133
1134This can also be enabled from the command line using the C<-C> switch.
1135
1136The initial value is typically C<0> for compatibility with Perl versions
1137earlier than 5.6, but may be automatically set to C<1> by Perl if the system
1138provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
1139
8058d7ab
GS
1140The C<bytes> pragma always overrides the effect of this flag in the current
1141lexical scope. See L<bytes>.
46487f74 1142
a0d0e21e
LW
1143=item $EXECUTABLE_NAME
1144
1145=item $^X
1146
1147The name that the Perl binary itself was executed as, from C's C<argv[0]>.
19799a22 1148This may not be a full pathname, nor even necessarily in your path.
a0d0e21e 1149
2d84a16a
DM
1150=item ARGV
1151
1152The special filehandle that iterates over command-line filenames in
1153C<@ARGV>. Usually written as the null filehandle in the angle operator
1154C<< <> >>. Note that currently C<ARGV> only has its magical effect
1155within the C<< <> >> operator; elsewhere it is just a plain filehandle
1156corresponding to the last file opened by C<< <> >>. In particular,
1157passing C<\*ARGV> as a parameter to a function that expects a filehandle
1158may not cause your function to automatically read the contents of all the
1159files in C<@ARGV>.
1160
a0d0e21e
LW
1161=item $ARGV
1162
c47ff5f1 1163contains the name of the current file when reading from <>.
a0d0e21e
LW
1164
1165=item @ARGV
1166
19799a22 1167The array @ARGV contains the command-line arguments intended for
14218588 1168the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1169one, because C<$ARGV[0]> is the first argument, I<not> the program's
1170command name itself. See C<$0> for the command name.
a0d0e21e 1171
9b0e6e7a
JP
1172=item @F
1173
1174The array @F contains the fields of each line read in when autosplit
1175mode is turned on. See L<perlrun> for the B<-a> switch. This array
1176is package-specific, and must be declared or given a full package name
1177if not in package main when running under C<strict 'vars'>.
1178
a0d0e21e
LW
1179=item @INC
1180
19799a22
GS
1181The array @INC contains the list of places that the C<do EXPR>,
1182C<require>, or C<use> constructs look for their library files. It
1183initially consists of the arguments to any B<-I> command-line
1184switches, followed by the default Perl library, probably
1185F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1186directory. ("." will not be appended if taint checks are enabled, either by
1187C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1188the C<use lib> pragma to get the machine-dependent library properly
1189loaded also:
a0d0e21e 1190
cb1a09d0
AD
1191 use lib '/mypath/libdir/';
1192 use SomeMod;
303f2f76 1193
d54b56d5
RGS
1194You can also insert hooks into the file inclusion system by putting Perl
1195code directly into @INC. Those hooks may be subroutine references, array
1196references or blessed objects. See L<perlfunc/require> for details.
1197
fb73857a
PP
1198=item @_
1199
1200Within a subroutine the array @_ contains the parameters passed to that
19799a22 1201subroutine. See L<perlsub>.
fb73857a 1202
a0d0e21e
LW
1203=item %INC
1204
19799a22
GS
1205The hash %INC contains entries for each filename included via the
1206C<do>, C<require>, or C<use> operators. The key is the filename
1207you specified (with module names converted to pathnames), and the
14218588 1208value is the location of the file found. The C<require>
87275199 1209operator uses this hash to determine whether a particular file has
19799a22 1210already been included.
a0d0e21e 1211
89ccab8c
RGS
1212If the file was loaded via a hook (e.g. a subroutine reference, see
1213L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1214by default inserted into %INC in place of a filename. Note, however,
1215that the hook may have set the %INC entry by itself to provide some more
1216specific info.
44f0be63 1217
b687b08b
TC
1218=item %ENV
1219
1220=item $ENV{expr}
a0d0e21e
LW
1221
1222The hash %ENV contains your current environment. Setting a
19799a22
GS
1223value in C<ENV> changes the environment for any child processes
1224you subsequently fork() off.
a0d0e21e 1225
b687b08b
TC
1226=item %SIG
1227
1228=item $SIG{expr}
a0d0e21e 1229
14218588 1230The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
1231
1232 sub handler { # 1st argument is signal name
fb73857a 1233 my($sig) = @_;
a0d0e21e
LW
1234 print "Caught a SIG$sig--shutting down\n";
1235 close(LOG);
1236 exit(0);
1237 }
1238
fb73857a
PP
1239 $SIG{'INT'} = \&handler;
1240 $SIG{'QUIT'} = \&handler;
a0d0e21e 1241 ...
19799a22 1242 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1243 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1244
f648820c
GS
1245Using a value of C<'IGNORE'> usually has the effect of ignoring the
1246signal, except for the C<CHLD> signal. See L<perlipc> for more about
1247this special case.
1248
19799a22 1249Here are some other examples:
a0d0e21e 1250
fb73857a 1251 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1252 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1253 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1254 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1255
19799a22
GS
1256Be sure not to use a bareword as the name of a signal handler,
1257lest you inadvertently call it.
748a9306 1258
44a8e56a
PP
1259If your system has the sigaction() function then signal handlers are
1260installed using it. This means you get reliable signal handling. If
1261your system has the SA_RESTART flag it is used when signals handlers are
19799a22 1262installed. This means that system calls for which restarting is supported
44a8e56a
PP
1263continue rather than returning when a signal arrives. If you want your
1264system calls to be interrupted by signal delivery then do something like
1265this:
1266
1267 use POSIX ':signal_h';
1268
1269 my $alarm = 0;
1270 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1271 or die "Error setting SIGALRM handler: $!\n";
1272
1273See L<POSIX>.
1274
748a9306 1275Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1276routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
1277about to be printed. The warning message is passed as the first
1278argument. The presence of a __WARN__ hook causes the ordinary printing
1279of warnings to STDERR to be suppressed. You can use this to save warnings
1280in a variable, or turn warnings into fatal errors, like this:
1281
1282 local $SIG{__WARN__} = sub { die $_[0] };
1283 eval $proggie;
1284
a8f8344d 1285The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
1286is about to be thrown. The error message is passed as the first
1287argument. When a __DIE__ hook routine returns, the exception
1288processing continues as it would have in the absence of the hook,
cb1a09d0 1289unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1290The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1291can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1292
19799a22
GS
1293Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1294even inside an eval(). Do not use this to rewrite a pending exception
1295in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1296This strange action at a distance may be fixed in a future release
1297so that C<$SIG{__DIE__}> is only called if your program is about
1298to exit, as was the original intent. Any other use is deprecated.
1299
1300C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1301they may be called to report (probable) errors found by the parser.
1302In such a case the parser may be in inconsistent state, so any
1303attempt to evaluate Perl code from such a handler will probably
1304result in a segfault. This means that warnings or errors that
1305result from parsing Perl should be used with extreme caution, like
1306this:
fb73857a
PP
1307
1308 require Carp if defined $^S;
1309 Carp::confess("Something wrong") if defined &Carp::confess;
1310 die "Something wrong, but could not load Carp to give backtrace...
1311 To see backtrace try starting Perl with -MCarp switch";
1312
1313Here the first line will load Carp I<unless> it is the parser who
1314called the handler. The second line will print backtrace and die if
1315Carp was available. The third line will be executed only if Carp was
1316not available.
1317
19799a22 1318See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1319L<warnings> for additional information.
68dc0745 1320
a0d0e21e 1321=back
55602bd2
IZ
1322
1323=head2 Error Indicators
1324
19799a22
GS
1325The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1326about different types of error conditions that may appear during
1327execution of a Perl program. The variables are shown ordered by
1328the "distance" between the subsystem which reported the error and
1329the Perl process. They correspond to errors detected by the Perl
1330interpreter, C library, operating system, or an external program,
1331respectively.
55602bd2
IZ
1332
1333To illustrate the differences between these variables, consider the
19799a22 1334following Perl expression, which uses a single-quoted string:
55602bd2 1335
19799a22 1336 eval q{
22d0716c
SB
1337 open my $pipe, "/cdrom/install |" or die $!;
1338 my @res = <$pipe>;
1339 close $pipe or die "bad pipe: $?, $!";
19799a22 1340 };
55602bd2
IZ
1341
1342After execution of this statement all 4 variables may have been set.
1343
19799a22
GS
1344C<$@> is set if the string to be C<eval>-ed did not compile (this
1345may happen if C<open> or C<close> were imported with bad prototypes),
1346or if Perl code executed during evaluation die()d . In these cases
1347the value of $@ is the compile error, or the argument to C<die>
1348(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1349though.)
1350
c47ff5f1 1351When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1352and C<close> are translated to calls in the C run-time library and
1353thence to the operating system kernel. C<$!> is set to the C library's
1354C<errno> if one of these calls fails.
1355
1356Under a few operating systems, C<$^E> may contain a more verbose
1357error indicator, such as in this case, "CDROM tray not closed."
14218588 1358Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1359the same as C<$!>.
1360
1361Finally, C<$?> may be set to non-0 value if the external program
1362F</cdrom/install> fails. The upper eight bits reflect specific
1363error conditions encountered by the program (the program's exit()
1364value). The lower eight bits reflect mode of failure, like signal
1365death and core dump information See wait(2) for details. In
1366contrast to C<$!> and C<$^E>, which are set only if error condition
1367is detected, the variable C<$?> is set on each C<wait> or pipe
1368C<close>, overwriting the old value. This is more like C<$@>, which
1369on every eval() is always set on failure and cleared on success.
2b92dfce 1370
19799a22
GS
1371For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1372and C<$?>.
2b92dfce
GS
1373
1374=head2 Technical Note on the Syntax of Variable Names
1375
19799a22
GS
1376Variable names in Perl can have several formats. Usually, they
1377must begin with a letter or underscore, in which case they can be
1378arbitrarily long (up to an internal limit of 251 characters) and
1379may contain letters, digits, underscores, or the special sequence
1380C<::> or C<'>. In this case, the part before the last C<::> or
1381C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1382
1383Perl variable names may also be a sequence of digits or a single
1384punctuation or control character. These names are all reserved for
19799a22
GS
1385special uses by Perl; for example, the all-digits names are used
1386to hold data captured by backreferences after a regular expression
1387match. Perl has a special syntax for the single-control-character
1388names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1389character. For example, the notation C<$^W> (dollar-sign caret
1390C<W>) is the scalar variable whose name is the single character
1391control-C<W>. This is better than typing a literal control-C<W>
1392into your program.
2b92dfce 1393
87275199 1394Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1395strings that begin with control characters (or better yet, a caret).
1396These variables must be written in the form C<${^Foo}>; the braces
1397are not optional. C<${^Foo}> denotes the scalar variable whose
1398name is a control-C<F> followed by two C<o>'s. These variables are
1399reserved for future special uses by Perl, except for the ones that
1400begin with C<^_> (control-underscore or caret-underscore). No
1401control-character name that begins with C<^_> will acquire a special
1402meaning in any future version of Perl; such names may therefore be
1403used safely in programs. C<$^_> itself, however, I<is> reserved.
1404
1405Perl identifiers that begin with digits, control characters, or
2b92dfce
GS
1406punctuation characters are exempt from the effects of the C<package>
1407declaration and are always forced to be in package C<main>. A few
1408other names are also exempt:
1409
1410 ENV STDIN
1411 INC STDOUT
1412 ARGV STDERR
1413 ARGVOUT
1414 SIG
1415
1416In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1417to be in package C<main>, regardless of any C<package> declarations
2b92dfce
GS
1418presently in scope.
1419
19799a22
GS
1420=head1 BUGS
1421
1422Due to an unfortunate accident of Perl's implementation, C<use
1423English> imposes a considerable performance penalty on all regular
1424expression matches in a program, regardless of whether they occur
1425in the scope of C<use English>. For that reason, saying C<use
1426English> in libraries is strongly discouraged. See the
1427Devel::SawAmpersand module documentation from CPAN
a93751fa 1428(http://www.cpan.org/modules/by-module/Devel/)
19799a22 1429for more information.
2b92dfce 1430
19799a22
GS
1431Having to even think about the C<$^S> variable in your exception
1432handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1433invites grievous and difficult to track down errors. Avoid it
1434and use an C<END{}> or CORE::GLOBAL::die override instead.