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1=head1 NAME
2
cb1a09d0 3perldata - Perl data types
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4
5=head1 DESCRIPTION
6
7=head2 Variable names
d74e8afc 8X<variable, name> X<variable name> X<data type> X<type>
a0d0e21e 9
d55a8828 10Perl has three built-in data types: scalars, arrays of scalars, and
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11associative arrays of scalars, known as "hashes". A scalar is a
12single string (of any size, limited only by the available memory),
13number, or a reference to something (which will be discussed
14in L<perlref>). Normal arrays are ordered lists of scalars indexed
15by number, starting with 0. Hashes are unordered collections of scalar
16values indexed by their associated string key.
a0d0e21e 17
d55a8828 18Values are usually referred to by name, or through a named reference.
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19The first character of the name tells you to what sort of data
20structure it refers. The rest of the name tells you the particular
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21value to which it refers. Usually this name is a single I<identifier>,
22that is, a string beginning with a letter or underscore, and
23containing letters, underscores, and digits. In some cases, it may
24be a chain of identifiers, separated by C<::> (or by the slightly
25archaic C<'>); all but the last are interpreted as names of packages,
26to locate the namespace in which to look up the final identifier
27(see L<perlmod/Packages> for details). It's possible to substitute
28for a simple identifier, an expression that produces a reference
29to the value at runtime. This is described in more detail below
30and in L<perlref>.
d74e8afc 31X<identifier>
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32
33Perl also has its own built-in variables whose names don't follow
34these rules. They have strange names so they don't accidentally
35collide with one of your normal variables. Strings that match
36parenthesized parts of a regular expression are saved under names
37containing only digits after the C<$> (see L<perlop> and L<perlre>).
38In addition, several special variables that provide windows into
39the inner working of Perl have names containing punctuation characters
40and control characters. These are documented in L<perlvar>.
d74e8afc 41X<variable, built-in>
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42
43Scalar values are always named with '$', even when referring to a
44scalar that is part of an array or a hash. The '$' symbol works
45semantically like the English word "the" in that it indicates a
46single value is expected.
d74e8afc 47X<scalar>
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48
49 $days # the simple scalar value "days"
50 $days[28] # the 29th element of array @days
51 $days{'Feb'} # the 'Feb' value from hash %days
52 $#days # the last index of array @days
53
d55a8828 54Entire arrays (and slices of arrays and hashes) are denoted by '@',
3921068c 55which works much as the word "these" or "those" does in English,
d55a8828 56in that it indicates multiple values are expected.
d74e8afc 57X<array>
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58
59 @days # ($days[0], $days[1],... $days[n])
d55a8828 60 @days[3,4,5] # same as ($days[3],$days[4],$days[5])
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61 @days{'a','c'} # same as ($days{'a'},$days{'c'})
62
d55a8828 63Entire hashes are denoted by '%':
d74e8afc 64X<hash>
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65
66 %days # (key1, val1, key2, val2 ...)
67
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68In addition, subroutines are named with an initial '&', though this
69is optional when unambiguous, just as the word "do" is often redundant
70in English. Symbol table entries can be named with an initial '*',
71but you don't really care about that yet (if ever :-).
72
73Every variable type has its own namespace, as do several
74non-variable identifiers. This means that you can, without fear
75of conflict, use the same name for a scalar variable, an array, or
76a hash--or, for that matter, for a filehandle, a directory handle, a
77subroutine name, a format name, or a label. This means that $foo
78and @foo are two different variables. It also means that C<$foo[1]>
79is a part of @foo, not a part of $foo. This may seem a bit weird,
80but that's okay, because it is weird.
d74e8afc 81X<namespace>
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82
83Because variable references always start with '$', '@', or '%', the
84"reserved" words aren't in fact reserved with respect to variable
85names. They I<are> reserved with respect to labels and filehandles,
86however, which don't have an initial special character. You can't
87have a filehandle named "log", for instance. Hint: you could say
88C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
89uppercase filehandles also improves readability and protects you
90from conflict with future reserved words. Case I<is> significant--"FOO",
91"Foo", and "foo" are all different names. Names that start with a
92letter or underscore may also contain digits and underscores.
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93X<identifier, case sensitivity>
94X<case>
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95
96It is possible to replace such an alphanumeric name with an expression
d55a8828 97that returns a reference to the appropriate type. For a description
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98of this, see L<perlref>.
99
5f05dabc 100Names that start with a digit may contain only more digits. Names
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101that do not start with a letter, underscore, digit or a caret (i.e.
102a control character) are limited to one character, e.g., C<$%> or
103C<$$>. (Most of these one character names have a predefined
104significance to Perl. For instance, C<$$> is the current process
105id.)
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106
107=head2 Context
d74e8afc 108X<context> X<scalar context> X<list context>
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109
110The interpretation of operations and values in Perl sometimes depends
111on the requirements of the context around the operation or value.
d55a8828 112There are two major contexts: list and scalar. Certain operations
a0d0e21e 113return list values in contexts wanting a list, and scalar values
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114otherwise. If this is true of an operation it will be mentioned in
115the documentation for that operation. In other words, Perl overloads
a0d0e21e 116certain operations based on whether the expected return value is
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117singular or plural. Some words in English work this way, like "fish"
118and "sheep".
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119
120In a reciprocal fashion, an operation provides either a scalar or a
121list context to each of its arguments. For example, if you say
122
123 int( <STDIN> )
124
c47ff5f1 125the integer operation provides scalar context for the <>
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126operator, which responds by reading one line from STDIN and passing it
127back to the integer operation, which will then find the integer value
128of that line and return that. If, on the other hand, you say
129
130 sort( <STDIN> )
131
c47ff5f1 132then the sort operation provides list context for <>, which
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133will proceed to read every line available up to the end of file, and
134pass that list of lines back to the sort routine, which will then
135sort those lines and return them as a list to whatever the context
136of the sort was.
137
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138Assignment is a little bit special in that it uses its left argument
139to determine the context for the right argument. Assignment to a
140scalar evaluates the right-hand side in scalar context, while
141assignment to an array or hash evaluates the righthand side in list
142context. Assignment to a list (or slice, which is just a list
3921068c 143anyway) also evaluates the right-hand side in list context.
d55a8828 144
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145When you use the C<use warnings> pragma or Perl's B<-w> command-line
146option, you may see warnings
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147about useless uses of constants or functions in "void context".
148Void context just means the value has been discarded, such as a
149statement containing only C<"fred";> or C<getpwuid(0);>. It still
150counts as scalar context for functions that care whether or not
151they're being called in list context.
152
153User-defined subroutines may choose to care whether they are being
154called in a void, scalar, or list context. Most subroutines do not
155need to bother, though. That's because both scalars and lists are
156automatically interpolated into lists. See L<perlfunc/wantarray>
157for how you would dynamically discern your function's calling
158context.
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159
160=head2 Scalar values
d74e8afc 161X<scalar> X<number> X<string> X<reference>
a0d0e21e 162
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163All data in Perl is a scalar, an array of scalars, or a hash of
164scalars. A scalar may contain one single value in any of three
165different flavors: a number, a string, or a reference. In general,
166conversion from one form to another is transparent. Although a
167scalar may not directly hold multiple values, it may contain a
168reference to an array or hash which in turn contains multiple values.
169
170Scalars aren't necessarily one thing or another. There's no place
171to declare a scalar variable to be of type "string", type "number",
172type "reference", or anything else. Because of the automatic
173conversion of scalars, operations that return scalars don't need
174to care (and in fact, cannot care) whether their caller is looking
175for a string, a number, or a reference. Perl is a contextually
176polymorphic language whose scalars can be strings, numbers, or
177references (which includes objects). Although strings and numbers
178are considered pretty much the same thing for nearly all purposes,
179references are strongly-typed, uncastable pointers with builtin
180reference-counting and destructor invocation.
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181
182A scalar value is interpreted as TRUE in the Boolean sense if it is not
19799a22 183the null string or the number 0 (or its string equivalent, "0"). The
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184Boolean context is just a special kind of scalar context where no
185conversion to a string or a number is ever performed.
d74e8afc 186X<boolean> X<bool> X<true> X<false> X<truth>
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187
188There are actually two varieties of null strings (sometimes referred
189to as "empty" strings), a defined one and an undefined one. The
190defined version is just a string of length zero, such as C<"">.
191The undefined version is the value that indicates that there is
192no real value for something, such as when there was an error, or
193at end of file, or when you refer to an uninitialized variable or
194element of an array or hash. Although in early versions of Perl,
195an undefined scalar could become defined when first used in a
196place expecting a defined value, this no longer happens except for
197rare cases of autovivification as explained in L<perlref>. You can
198use the defined() operator to determine whether a scalar value is
199defined (this has no meaning on arrays or hashes), and the undef()
200operator to produce an undefined value.
d74e8afc 201X<defined> X<undefined> X<undef> X<null> X<string, null>
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202
203To find out whether a given string is a valid non-zero number, it's
204sometimes enough to test it against both numeric 0 and also lexical
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205"0" (although this will cause noises if warnings are on). That's
206because strings that aren't numbers count as 0, just as they do in B<awk>:
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207
208 if ($str == 0 && $str ne "0") {
209 warn "That doesn't look like a number";
54310121 210 }
4633a7c4 211
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212That method may be best because otherwise you won't treat IEEE
213notations like C<NaN> or C<Infinity> properly. At other times, you
214might prefer to determine whether string data can be used numerically
215by calling the POSIX::strtod() function or by inspecting your string
216with a regular expression (as documented in L<perlre>).
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217
218 warn "has nondigits" if /\D/;
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219 warn "not a natural number" unless /^\d+$/; # rejects -3
220 warn "not an integer" unless /^-?\d+$/; # rejects +3
221 warn "not an integer" unless /^[+-]?\d+$/;
222 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
223 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
54310121 224 warn "not a C float"
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225 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
226
d55a8828 227The length of an array is a scalar value. You may find the length
fc518ee5 228of array @days by evaluating C<$#days>, as in B<csh>. However, this
229isn't the length of the array; it's the subscript of the last element,
230which is a different value since there is ordinarily a 0th element.
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231Assigning to C<$#days> actually changes the length of the array.
232Shortening an array this way destroys intervening values. Lengthening
233an array that was previously shortened does not recover values
234that were in those elements. (It used to do so in Perl 4, but we
235had to break this to make sure destructors were called when expected.)
d74e8afc 236X<$#> X<array, length>
d55a8828 237
210b36aa 238You can also gain some minuscule measure of efficiency by pre-extending
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239an array that is going to get big. You can also extend an array
240by assigning to an element that is off the end of the array. You
19799a22 241can truncate an array down to nothing by assigning the null list
d55a8828 242() to it. The following are equivalent:
a0d0e21e 243
84f709e7 244 @whatever = ();
3e3baf6d 245 $#whatever = -1;
a0d0e21e 246
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247If you evaluate an array in scalar context, it returns the length
248of the array. (Note that this is not true of lists, which return
249the last value, like the C comma operator, nor of built-in functions,
250which return whatever they feel like returning.) The following is
251always true:
d74e8afc 252X<array, length>
a0d0e21e 253
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254 scalar(@whatever) == $#whatever - $[ + 1;
255
256Version 5 of Perl changed the semantics of C<$[>: files that don't set
257the value of C<$[> no longer need to worry about whether another
258file changed its value. (In other words, use of C<$[> is deprecated.)
259So in general you can assume that
d74e8afc 260X<$[>
84f709e7 261
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262 scalar(@whatever) == $#whatever + 1;
263
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264Some programmers choose to use an explicit conversion so as to
265leave nothing to doubt:
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266
267 $element_count = scalar(@whatever);
268
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269If you evaluate a hash in scalar context, it returns false if the
270hash is empty. If there are any key/value pairs, it returns true;
271more precisely, the value returned is a string consisting of the
272number of used buckets and the number of allocated buckets, separated
273by a slash. This is pretty much useful only to find out whether
274Perl's internal hashing algorithm is performing poorly on your data
275set. For example, you stick 10,000 things in a hash, but evaluating
276%HASH in scalar context reveals C<"1/16">, which means only one out
277of sixteen buckets has been touched, and presumably contains all
126c71c8 27810,000 of your items. This isn't supposed to happen. If a tied hash
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279is evaluated in scalar context, the C<SCALAR> method is called (with a
280fallback to C<FIRSTKEY>).
d74e8afc 281X<hash, scalar context> X<hash, bucket> X<bucket>
a0d0e21e 282
5a964f20 283You can preallocate space for a hash by assigning to the keys() function.
65841adf 284This rounds up the allocated buckets to the next power of two:
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285
286 keys(%users) = 1000; # allocate 1024 buckets
287
a0d0e21e 288=head2 Scalar value constructors
d74e8afc 289X<scalar, literal> X<scalar, constant>
a0d0e21e 290
d55a8828 291Numeric literals are specified in any of the following floating point or
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292integer formats:
293
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294 12345
295 12345.67
d55a8828 296 .23E-10 # a very small number
928753ea 297 3.14_15_92 # a very important number
1d277562 298 4_294_967_296 # underscore for legibility
d55a8828 299 0xff # hex
928753ea 300 0xdead_beef # more hex
802a55ac 301 0377 # octal (only numbers, begins with 0)
d55a8828 302 0b011011 # binary
a0d0e21e 303
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304You are allowed to use underscores (underbars) in numeric literals
305between digits for legibility. You could, for example, group binary
306digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
307or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
d74e8afc 308X<number, literal>
1d277562 309
55497cff 310String literals are usually delimited by either single or double
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311quotes. They work much like quotes in the standard Unix shells:
312double-quoted string literals are subject to backslash and variable
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313substitution; single-quoted strings are not (except for C<\'> and
314C<\\>). The usual C-style backslash rules apply for making
d55a8828 315characters such as newline, tab, etc., as well as some more exotic
4a4eefd0 316forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
d74e8afc 317X<string, literal>
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318
319Hexadecimal, octal, or binary, representations in string literals
320(e.g. '0xff') are not automatically converted to their integer
321representation. The hex() and oct() functions make these conversions
322for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
68dc0745 323
5f05dabc 324You can also embed newlines directly in your strings, i.e., they can end
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325on a different line than they begin. This is nice, but if you forget
326your trailing quote, the error will not be reported until Perl finds
327another line containing the quote character, which may be much further
328on in the script. Variable substitution inside strings is limited to
d55a8828 329scalar variables, arrays, and array or hash slices. (In other words,
b88cefa9 330names beginning with $ or @, followed by an optional bracketed
a0d0e21e 331expression as a subscript.) The following code segment prints out "The
184e9718 332price is $Z<>100."
d74e8afc 333X<interpolation>
a0d0e21e 334
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335 $Price = '$100'; # not interpolated
336 print "The price is $Price.\n"; # interpolated
337
338There is no double interpolation in Perl, so the C<$100> is left as is.
a0d0e21e 339
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340By default floating point numbers substituted inside strings use the
341dot (".") as the decimal separator. If C<use locale> is in effect,
342and POSIX::setlocale() has been called, the character used for the
343decimal separator is affected by the LC_NUMERIC locale.
344See L<perllocale> and L<POSIX>.
345
d55a8828 346As in some shells, you can enclose the variable name in braces to
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347disambiguate it from following alphanumerics (and underscores).
348You must also do
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349this when interpolating a variable into a string to separate the
350variable name from a following double-colon or an apostrophe, since
351these would be otherwise treated as a package separator:
d74e8afc 352X<interpolation>
d55a8828 353
84f709e7 354 $who = "Larry";
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355 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
356 print "We use ${who}speak when ${who}'s here.\n";
357
358Without the braces, Perl would have looked for a $whospeak, a
359C<$who::0>, and a C<$who's> variable. The last two would be the
360$0 and the $s variables in the (presumably) non-existent package
361C<who>.
362
363In fact, an identifier within such curlies is forced to be a string,
364as is any simple identifier within a hash subscript. Neither need
365quoting. Our earlier example, C<$days{'Feb'}> can be written as
366C<$days{Feb}> and the quotes will be assumed automatically. But
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367anything more complicated in the subscript will be interpreted as an
368expression. This means for example that C<$version{2.0}++> is
369equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
d55a8828 370
692ef166 371=head3 Version Strings
d74e8afc 372X<version string> X<vstring> X<v-string>
692ef166 373
191d61a7 374A literal of the form C<v1.20.300.4000> is parsed as a string composed
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375of characters with the specified ordinals. This form, known as
376v-strings, provides an alternative, more readable way to construct
377strings, rather than use the somewhat less readable interpolation form
378C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
379Unicode strings, and for comparing version "numbers" using the string
380comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
381more dots in the literal, the leading C<v> may be omitted.
b9c62f5b 382
2575c402 383 print v9786; # prints SMILEY, "\x{263a}"
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384 print v102.111.111; # prints "foo"
385 print 102.111.111; # same
386
387Such literals are accepted by both C<require> and C<use> for
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388doing a version check. Note that using the v-strings for IPv4
389addresses is not portable unless you also use the
390inet_aton()/inet_ntoa() routines of the Socket package.
191d61a7 391
d32a65d2 392Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
8fa72689 393are not v-strings before the C<< => >> operator (which is usually used
3921068c 394to separate a hash key from a hash value); instead they are interpreted
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395as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
396Perl 5.8.0, but that caused more confusion and breakage than good.
397Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
398be v-strings always.
d32a65d2 399
692ef166 400=head3 Special Literals
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401X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
402X<end> X<data> X<^D> X<^Z>
692ef166 403
d55a8828 404The special literals __FILE__, __LINE__, and __PACKAGE__
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405represent the current filename, line number, and package name at that
406point in your program. They may be used only as separate tokens; they
407will not be interpolated into strings. If there is no current package
3e92a254 408(due to an empty C<package;> directive), __PACKAGE__ is the undefined
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409value. (But the empty C<package;> is no longer supported, as of version
4105.10.)
d74e8afc 411X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
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412
413The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
414may be used to indicate the logical end of the script before the actual
415end of file. Any following text is ignored.
416
1bab44f9 417Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
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418where C<PACKNAME> is the package that was current when the __DATA__
419token was encountered. The filehandle is left open pointing to the
bbc73fd9 420line after __DATA__. It is the program's responsibility to
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421C<close DATA> when it is done reading from it. For compatibility with
422older scripts written before __DATA__ was introduced, __END__ behaves
353c6505 423like __DATA__ in the top level script (but not in files loaded with
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424C<require> or C<do>) and leaves the remaining contents of the
425file accessible via C<main::DATA>.
426
427See L<SelfLoader> for more description of __DATA__, and
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428an example of its use. Note that you cannot read from the DATA
429filehandle in a BEGIN block: the BEGIN block is executed as soon
430as it is seen (during compilation), at which point the corresponding
a00c1fe5 431__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 432
692ef166 433=head3 Barewords
d74e8afc 434X<bareword>
692ef166 435
748a9306 436A word that has no other interpretation in the grammar will
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437be treated as if it were a quoted string. These are known as
438"barewords". As with filehandles and labels, a bareword that consists
439entirely of lowercase letters risks conflict with future reserved
9f1b1f2d 440words, and if you use the C<use warnings> pragma or the B<-w> switch,
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441Perl will warn you about any such words. Perl limits barewords (like
442identifiers) to about 250 characters. Future versions of Perl are likely
443to eliminate these arbitrary limitations.
444
445Some people may wish to outlaw barewords entirely. If you
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446say
447
448 use strict 'subs';
449
450then any bareword that would NOT be interpreted as a subroutine call
451produces a compile-time error instead. The restriction lasts to the
54310121 452end of the enclosing block. An inner block may countermand this
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453by saying C<no strict 'subs'>.
454
e2b457c0 455=head3 Array Interpolation
d74e8afc 456X<array, interpolation> X<interpolation, array> X<$">
692ef166 457
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458Arrays and slices are interpolated into double-quoted strings
459by joining the elements with the delimiter specified in the C<$">
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460variable (C<$LIST_SEPARATOR> if "use English;" is specified),
461space by default. The following are equivalent:
a0d0e21e 462
84f709e7 463 $temp = join($", @ARGV);
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464 system "echo $temp";
465
466 system "echo @ARGV";
467
468Within search patterns (which also undergo double-quotish substitution)
d55a8828 469there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
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470C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
471expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
472@foo)? If @foo doesn't otherwise exist, then it's obviously a
473character class. If @foo exists, Perl takes a good guess about C<[bar]>,
474and is almost always right. If it does guess wrong, or if you're just
475plain paranoid, you can force the correct interpretation with curly
d55a8828 476braces as above.
a0d0e21e 477
7e3b091d 478If you're looking for the information on how to use here-documents,
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AMS
479which used to be here, that's been moved to
480L<perlop/Quote and Quote-like Operators>.
be16fac9 481
a0d0e21e 482=head2 List value constructors
d74e8afc 483X<list>
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484
485List values are denoted by separating individual values by commas
486(and enclosing the list in parentheses where precedence requires it):
487
488 (LIST)
489
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490In a context not requiring a list value, the value of what appears
491to be a list literal is simply the value of the final element, as
492with the C comma operator. For example,
a0d0e21e 493
84f709e7 494 @foo = ('cc', '-E', $bar);
a0d0e21e 495
d55a8828 496assigns the entire list value to array @foo, but
a0d0e21e 497
84f709e7 498 $foo = ('cc', '-E', $bar);
a0d0e21e 499
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500assigns the value of variable $bar to the scalar variable $foo.
501Note that the value of an actual array in scalar context is the
502length of the array; the following assigns the value 3 to $foo:
a0d0e21e 503
84f709e7 504 @foo = ('cc', '-E', $bar);
7e3b091d 505 $foo = @foo; # $foo gets 3
a0d0e21e 506
54310121 507You may have an optional comma before the closing parenthesis of a
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508list literal, so that you can say:
509
84f709e7 510 @foo = (
7e3b091d
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511 1,
512 2,
513 3,
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514 );
515
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516To use a here-document to assign an array, one line per element,
517you might use an approach like this:
518
84f709e7 519 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
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520 normal tomato
521 spicy tomato
522 green chile
523 pesto
524 white wine
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525 End_Lines
526
a0d0e21e 527LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 528evaluated, each element of the list is evaluated in list context, and
a0d0e21e 529the resulting list value is interpolated into LIST just as if each
5a964f20 530individual element were a member of LIST. Thus arrays and hashes lose their
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LW
531identity in a LIST--the list
532
5a964f20 533 (@foo,@bar,&SomeSub,%glarch)
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534
535contains all the elements of @foo followed by all the elements of @bar,
5a964f20 536followed by all the elements returned by the subroutine named SomeSub
d55a8828 537called in list context, followed by the key/value pairs of %glarch.
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538To make a list reference that does I<NOT> interpolate, see L<perlref>.
539
19799a22 540The null list is represented by (). Interpolating it in a list
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LW
541has no effect. Thus ((),(),()) is equivalent to (). Similarly,
542interpolating an array with no elements is the same as if no
543array had been interpolated at that point.
544
c2689353 545This interpolation combines with the facts that the opening
ab1f959b 546and closing parentheses are optional (except when necessary for
c2689353
NC
547precedence) and lists may end with an optional comma to mean that
548multiple commas within lists are legal syntax. The list C<1,,3> is a
549concatenation of two lists, C<1,> and C<3>, the first of which ends
550with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
551similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
552we'd advise you to use this obfuscation.
553
a0d0e21e 554A list value may also be subscripted like a normal array. You must
54310121 555put the list in parentheses to avoid ambiguity. For example:
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556
557 # Stat returns list value.
84f709e7 558 $time = (stat($file))[8];
a0d0e21e 559
4633a7c4 560 # SYNTAX ERROR HERE.
84f709e7 561 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 562
a0d0e21e 563 # Find a hex digit.
84f709e7 564 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
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565
566 # A "reverse comma operator".
567 return (pop(@foo),pop(@foo))[0];
568
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569Lists may be assigned to only when each element of the list
570is itself legal to assign to:
a0d0e21e 571
84f709e7 572 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 573
84f709e7 574 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 575
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576An exception to this is that you may assign to C<undef> in a list.
577This is useful for throwing away some of the return values of a
578function:
579
84f709e7 580 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
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581
582List assignment in scalar context returns the number of elements
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LW
583produced by the expression on the right side of the assignment:
584
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DA
585 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
586 $x = (($foo,$bar) = f()); # set $x to f()'s return count
4633a7c4 587
d55a8828 588This is handy when you want to do a list assignment in a Boolean
19799a22 589context, because most list functions return a null list when finished,
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LW
590which when assigned produces a 0, which is interpreted as FALSE.
591
ab1f959b
PN
592It's also the source of a useful idiom for executing a function or
593performing an operation in list context and then counting the number of
594return values, by assigning to an empty list and then using that
595assignment in scalar context. For example, this code:
596
84f709e7 597 $count = () = $string =~ /\d+/g;
ab1f959b
PN
598
599will place into $count the number of digit groups found in $string.
600This happens because the pattern match is in list context (since it
601is being assigned to the empty list), and will therefore return a list
602of all matching parts of the string. The list assignment in scalar
603context will translate that into the number of elements (here, the
604number of times the pattern matched) and assign that to $count. Note
605that simply using
606
84f709e7 607 $count = $string =~ /\d+/g;
ab1f959b
PN
608
609would not have worked, since a pattern match in scalar context will
610only return true or false, rather than a count of matches.
611
612The final element of a list assignment may be an array or a hash:
a0d0e21e 613
84f709e7 614 ($a, $b, @rest) = split;
5a964f20 615 my($a, $b, %rest) = @_;
a0d0e21e 616
4633a7c4 617You can actually put an array or hash anywhere in the list, but the first one
d55a8828
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618in the list will soak up all the values, and anything after it will become
619undefined. This may be useful in a my() or local().
a0d0e21e 620
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621A hash can be initialized using a literal list holding pairs of
622items to be interpreted as a key and a value:
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LW
623
624 # same as map assignment above
84f709e7 625 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 626
d55a8828 627While literal lists and named arrays are often interchangeable, that's
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LW
628not the case for hashes. Just because you can subscript a list value like
629a normal array does not mean that you can subscript a list value as a
630hash. Likewise, hashes included as parts of other lists (including
631parameters lists and return lists from functions) always flatten out into
632key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 633
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634It is often more readable to use the C<< => >> operator between key/value
635pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 636synonym for a comma, but it also arranges for its left-hand operand to be
ac036724 637interpreted as a string if it's a bareword that would be a legal simple
638identifier. C<< => >> doesn't quote compound identifiers, that contain
639double colons. This makes it nice for initializing hashes:
a0d0e21e 640
84f709e7 641 %map = (
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642 red => 0x00f,
643 blue => 0x0f0,
644 green => 0xf00,
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LW
645 );
646
647or for initializing hash references to be used as records:
648
84f709e7 649 $rec = {
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650 witch => 'Mable the Merciless',
651 cat => 'Fluffy the Ferocious',
652 date => '10/31/1776',
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LW
653 };
654
655or for using call-by-named-parameter to complicated functions:
656
84f709e7 657 $field = $query->radio_group(
7e3b091d 658 name => 'group_name',
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LW
659 values => ['eenie','meenie','minie'],
660 default => 'meenie',
661 linebreak => 'true',
84f709e7 662 labels => \%labels
4633a7c4 663 );
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664
665Note that just because a hash is initialized in that order doesn't
666mean that it comes out in that order. See L<perlfunc/sort> for examples
667of how to arrange for an output ordering.
668
692ef166
SF
669=head2 Subscripts
670
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671An array can be accessed one scalar at a
672time by specifying a dollar sign (C<$>), then the
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SF
673name of the array (without the leading C<@>), then the subscript inside
674square brackets. For example:
675
676 @myarray = (5, 50, 500, 5000);
2adc35dd 677 print "The Third Element is", $myarray[2], "\n";
692ef166
SF
678
679The array indices start with 0. A negative subscript retrieves its
680value from the end. In our example, C<$myarray[-1]> would have been
6815000, and C<$myarray[-2]> would have been 500.
682
683Hash subscripts are similar, only instead of square brackets curly brackets
684are used. For example:
685
686 %scientists =
687 (
688 "Newton" => "Isaac",
689 "Einstein" => "Albert",
690 "Darwin" => "Charles",
691 "Feynman" => "Richard",
692 );
693
694 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
695
aa80e1dc 696You can also subscript a list to get a single element from it:
d55a8828 697
aa80e1dc 698 $dir = (getpwnam("daemon"))[7];
d55a8828 699
aa80e1dc
FC
700=head2 Slices
701X<slice> X<array, slice> X<hash, slice>
d55a8828
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702
703A slice accesses several elements of a list, an array, or a hash
56d7751a
GS
704simultaneously using a list of subscripts. It's more convenient
705than writing out the individual elements as a list of separate
d55a8828
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706scalar values.
707
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708 ($him, $her) = @folks[0,-1]; # array slice
709 @them = @folks[0 .. 3]; # array slice
710 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
711 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
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712
713Since you can assign to a list of variables, you can also assign to
714an array or hash slice.
715
84f709e7 716 @days[3..5] = qw/Wed Thu Fri/;
d55a8828 717 @colors{'red','blue','green'}
7e3b091d 718 = (0xff0000, 0x0000ff, 0x00ff00);
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TC
719 @folks[0, -1] = @folks[-1, 0];
720
721The previous assignments are exactly equivalent to
722
84f709e7
JH
723 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
724 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
7e3b091d 725 = (0xff0000, 0x0000ff, 0x00ff00);
88fd19e3 726 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
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727
728Since changing a slice changes the original array or hash that it's
56d7751a
GS
729slicing, a C<foreach> construct will alter some--or even all--of the
730values of the array or hash.
d55a8828
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731
732 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
733
00cb5da1 734 foreach (@hash{qw[key1 key2]}) {
7e3b091d
DA
735 s/^\s+//; # trim leading whitespace
736 s/\s+$//; # trim trailing whitespace
737 s/(\w+)/\u\L$1/g; # "titlecase" words
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738 }
739
08cd8952
GS
740A slice of an empty list is still an empty list. Thus:
741
84f709e7
JH
742 @a = ()[1,0]; # @a has no elements
743 @b = (@a)[0,1]; # @b has no elements
744 @c = (0,1)[2,3]; # @c has no elements
56d7751a
GS
745
746But:
747
84f709e7
JH
748 @a = (1)[1,0]; # @a has two elements
749 @b = (1,undef)[1,0,2]; # @b has three elements
08cd8952 750
19799a22
GS
751This makes it easy to write loops that terminate when a null list
752is returned:
d55a8828 753
84f709e7 754 while ( ($home, $user) = (getpwent)[7,0]) {
7e3b091d 755 printf "%-8s %s\n", $user, $home;
d55a8828
TC
756 }
757
758As noted earlier in this document, the scalar sense of list assignment
759is the number of elements on the right-hand side of the assignment.
19799a22 760The null list contains no elements, so when the password file is
d55a8828
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761exhausted, the result is 0, not 2.
762
ad1de9c6
ML
763Slices in scalar context return the last item of the slice.
764
765 @a = qw/first second third/;
766 %h = (first => 'A', second => 'B');
767 $t = @a[0, 1]; # $t is now 'second'
0de10106 768 $u = @h{'first', 'second'}; # $u is now 'B'
ad1de9c6 769
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770If you're confused about why you use an '@' there on a hash slice
771instead of a '%', think of it like this. The type of bracket (square
772or curly) governs whether it's an array or a hash being looked at.
773On the other hand, the leading symbol ('$' or '@') on the array or
774hash indicates whether you are getting back a singular value (a
775scalar) or a plural one (a list).
776
5f05dabc 777=head2 Typeglobs and Filehandles
d74e8afc 778X<typeglob> X<filehandle> X<*>
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779
780Perl uses an internal type called a I<typeglob> to hold an entire
781symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 782it represents all types. This used to be the preferred way to
cb1a09d0 783pass arrays and hashes by reference into a function, but now that
5a964f20
TC
784we have real references, this is seldom needed.
785
786The main use of typeglobs in modern Perl is create symbol table aliases.
787This assignment:
788
789 *this = *that;
790
791makes $this an alias for $that, @this an alias for @that, %this an alias
792for %that, &this an alias for &that, etc. Much safer is to use a reference.
793This:
5f05dabc 794
5a964f20
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795 local *Here::blue = \$There::green;
796
797temporarily makes $Here::blue an alias for $There::green, but doesn't
798make @Here::blue an alias for @There::green, or %Here::blue an alias for
799%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
800of this. Strange though this may seem, this is the basis for the whole
84f709e7 801module import/export system.
5a964f20 802
d55a8828 803Another use for typeglobs is to pass filehandles into a function or
5a964f20
TC
804to create new filehandles. If you need to use a typeglob to save away
805a filehandle, do it this way:
5f05dabc 806
84f709e7 807 $fh = *STDOUT;
5f05dabc
PP
808
809or perhaps as a real reference, like this:
810
84f709e7 811 $fh = \*STDOUT;
5f05dabc 812
5a964f20
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813See L<perlsub> for examples of using these as indirect filehandles
814in functions.
815
816Typeglobs are also a way to create a local filehandle using the local()
817operator. These last until their block is exited, but may be passed back.
818For example:
5f05dabc
PP
819
820 sub newopen {
7e3b091d
DA
821 my $path = shift;
822 local *FH; # not my!
823 open (FH, $path) or return undef;
824 return *FH;
5f05dabc 825 }
84f709e7 826 $fh = newopen('/etc/passwd');
5f05dabc 827
d55a8828 828Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
5a964f20
TC
829for filehandle manipulations, although they're still needed to pass brand
830new file and directory handles into or out of functions. That's because
d55a8828
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831C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
832In other words, C<*FH> must be used to create new symbol table entries;
833C<*foo{THING}> cannot. When in doubt, use C<*FH>.
834
36392fcf
GS
835All functions that are capable of creating filehandles (open(),
836opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
837automatically create an anonymous filehandle if the handle passed to
838them is an uninitialized scalar variable. This allows the constructs
839such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
840create filehandles that will conveniently be closed automatically when
841the scope ends, provided there are no other references to them. This
842largely eliminates the need for typeglobs when opening filehandles
843that must be passed around, as in the following example:
844
845 sub myopen {
84f709e7 846 open my $fh, "@_"
7e3b091d
DA
847 or die "Can't open '@_': $!";
848 return $fh;
36392fcf
GS
849 }
850
851 {
852 my $f = myopen("</etc/motd");
7e3b091d
DA
853 print <$f>;
854 # $f implicitly closed here
36392fcf
GS
855 }
856
b92795fe
AMS
857Note that if an initialized scalar variable is used instead the
858result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
859to C<open( *{'zzz'}, ...)>.
d83fe814
AT
860C<use strict 'refs'> forbids such practice.
861
d55a8828
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862Another way to create anonymous filehandles is with the Symbol
863module or with the IO::Handle module and its ilk. These modules
864have the advantage of not hiding different types of the same name
66b6e4ad
KW
865during the local(). See the bottom of L<perlfunc/open> for an
866example.
d55a8828
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867
868=head1 SEE ALSO
869
870See L<perlvar> for a description of Perl's built-in variables and
871a discussion of legal variable names. See L<perlref>, L<perlsub>,
872and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
873the C<*foo{THING}> syntax.