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RT #75870 perldata.pod tied hash in scalar context
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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.
b88cefa9 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 $#days # the last index of array @days
53
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54Entire arrays (and slices of arrays and hashes) are denoted by '@',
55which works much like the word "these" or "those" does in English,
56in that it indicates multiple values are expected.
d74e8afc 57X<array>
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58
59 @days # ($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
143anyway) also evaluates the righthand side in list context.
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+))?$/;
226
d55a8828 227The length of an array is a scalar value. You may find the length
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228of array @days by evaluating C<$#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>)
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393are not v-strings before the C<< => >> operator (which is usually used
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__
68dc0745 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
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408(due to an empty C<package;> directive), __PACKAGE__ is the undefined
409value.
d74e8afc 410X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
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411
412The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
413may be used to indicate the logical end of the script before the actual
414end of file. Any following text is ignored.
415
1bab44f9 416Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
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417where C<PACKNAME> is the package that was current when the __DATA__
418token was encountered. The filehandle is left open pointing to the
419contents after __DATA__. It is the program's responsibility to
420C<close DATA> when it is done reading from it. For compatibility with
421older scripts written before __DATA__ was introduced, __END__ behaves
353c6505 422like __DATA__ in the top level script (but not in files loaded with
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423C<require> or C<do>) and leaves the remaining contents of the
424file accessible via C<main::DATA>.
425
426See L<SelfLoader> for more description of __DATA__, and
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427an example of its use. Note that you cannot read from the DATA
428filehandle in a BEGIN block: the BEGIN block is executed as soon
429as it is seen (during compilation), at which point the corresponding
a00c1fe5 430__DATA__ (or __END__) token has not yet been seen.
a0d0e21e 431
692ef166 432=head3 Barewords
d74e8afc 433X<bareword>
692ef166 434
748a9306 435A word that has no other interpretation in the grammar will
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436be treated as if it were a quoted string. These are known as
437"barewords". As with filehandles and labels, a bareword that consists
438entirely of lowercase letters risks conflict with future reserved
9f1b1f2d 439words, and if you use the C<use warnings> pragma or the B<-w> switch,
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440Perl will warn you about any such words. Perl limits barewords (like
441identifiers) to about 250 characters. Future versions of Perl are likely
442to eliminate these arbitrary limitations.
443
444Some people may wish to outlaw barewords entirely. If you
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445say
446
447 use strict 'subs';
448
449then any bareword that would NOT be interpreted as a subroutine call
450produces a compile-time error instead. The restriction lasts to the
54310121 451end of the enclosing block. An inner block may countermand this
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452by saying C<no strict 'subs'>.
453
692ef166 454=head3 Array Joining Delimiter
d74e8afc 455X<array, interpolation> X<interpolation, array> X<$">
692ef166 456
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457Arrays and slices are interpolated into double-quoted strings
458by joining the elements with the delimiter specified in the C<$">
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459variable (C<$LIST_SEPARATOR> if "use English;" is specified),
460space by default. The following are equivalent:
a0d0e21e 461
84f709e7 462 $temp = join($", @ARGV);
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463 system "echo $temp";
464
465 system "echo @ARGV";
466
467Within search patterns (which also undergo double-quotish substitution)
d55a8828 468there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
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469C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
470expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
471@foo)? If @foo doesn't otherwise exist, then it's obviously a
472character class. If @foo exists, Perl takes a good guess about C<[bar]>,
473and is almost always right. If it does guess wrong, or if you're just
474plain paranoid, you can force the correct interpretation with curly
d55a8828 475braces as above.
a0d0e21e 476
7e3b091d 477If you're looking for the information on how to use here-documents,
210b36aa
AMS
478which used to be here, that's been moved to
479L<perlop/Quote and Quote-like Operators>.
be16fac9 480
a0d0e21e 481=head2 List value constructors
d74e8afc 482X<list>
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483
484List values are denoted by separating individual values by commas
485(and enclosing the list in parentheses where precedence requires it):
486
487 (LIST)
488
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489In a context not requiring a list value, the value of what appears
490to be a list literal is simply the value of the final element, as
491with the C comma operator. For example,
a0d0e21e 492
84f709e7 493 @foo = ('cc', '-E', $bar);
a0d0e21e 494
d55a8828 495assigns the entire list value to array @foo, but
a0d0e21e 496
84f709e7 497 $foo = ('cc', '-E', $bar);
a0d0e21e 498
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499assigns the value of variable $bar to the scalar variable $foo.
500Note that the value of an actual array in scalar context is the
501length of the array; the following assigns the value 3 to $foo:
a0d0e21e 502
84f709e7 503 @foo = ('cc', '-E', $bar);
7e3b091d 504 $foo = @foo; # $foo gets 3
a0d0e21e 505
54310121 506You may have an optional comma before the closing parenthesis of a
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507list literal, so that you can say:
508
84f709e7 509 @foo = (
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510 1,
511 2,
512 3,
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513 );
514
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515To use a here-document to assign an array, one line per element,
516you might use an approach like this:
517
84f709e7 518 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
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519 normal tomato
520 spicy tomato
521 green chile
522 pesto
523 white wine
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524 End_Lines
525
a0d0e21e 526LISTs do automatic interpolation of sublists. That is, when a LIST is
d55a8828 527evaluated, each element of the list is evaluated in list context, and
a0d0e21e 528the resulting list value is interpolated into LIST just as if each
5a964f20 529individual element were a member of LIST. Thus arrays and hashes lose their
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530identity in a LIST--the list
531
5a964f20 532 (@foo,@bar,&SomeSub,%glarch)
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533
534contains all the elements of @foo followed by all the elements of @bar,
5a964f20 535followed by all the elements returned by the subroutine named SomeSub
d55a8828 536called in list context, followed by the key/value pairs of %glarch.
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537To make a list reference that does I<NOT> interpolate, see L<perlref>.
538
19799a22 539The null list is represented by (). Interpolating it in a list
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LW
540has no effect. Thus ((),(),()) is equivalent to (). Similarly,
541interpolating an array with no elements is the same as if no
542array had been interpolated at that point.
543
c2689353 544This interpolation combines with the facts that the opening
ab1f959b 545and closing parentheses are optional (except when necessary for
c2689353
NC
546precedence) and lists may end with an optional comma to mean that
547multiple commas within lists are legal syntax. The list C<1,,3> is a
548concatenation of two lists, C<1,> and C<3>, the first of which ends
549with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
550similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
551we'd advise you to use this obfuscation.
552
a0d0e21e 553A list value may also be subscripted like a normal array. You must
54310121 554put the list in parentheses to avoid ambiguity. For example:
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555
556 # Stat returns list value.
84f709e7 557 $time = (stat($file))[8];
a0d0e21e 558
4633a7c4 559 # SYNTAX ERROR HERE.
84f709e7 560 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
4633a7c4 561
a0d0e21e 562 # Find a hex digit.
84f709e7 563 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
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564
565 # A "reverse comma operator".
566 return (pop(@foo),pop(@foo))[0];
567
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568Lists may be assigned to only when each element of the list
569is itself legal to assign to:
a0d0e21e 570
84f709e7 571 ($a, $b, $c) = (1, 2, 3);
a0d0e21e 572
84f709e7 573 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
a0d0e21e 574
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575An exception to this is that you may assign to C<undef> in a list.
576This is useful for throwing away some of the return values of a
577function:
578
84f709e7 579 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
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580
581List assignment in scalar context returns the number of elements
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LW
582produced by the expression on the right side of the assignment:
583
7e3b091d
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584 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
585 $x = (($foo,$bar) = f()); # set $x to f()'s return count
4633a7c4 586
d55a8828 587This is handy when you want to do a list assignment in a Boolean
19799a22 588context, because most list functions return a null list when finished,
4633a7c4
LW
589which when assigned produces a 0, which is interpreted as FALSE.
590
ab1f959b
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591It's also the source of a useful idiom for executing a function or
592performing an operation in list context and then counting the number of
593return values, by assigning to an empty list and then using that
594assignment in scalar context. For example, this code:
595
84f709e7 596 $count = () = $string =~ /\d+/g;
ab1f959b
PN
597
598will place into $count the number of digit groups found in $string.
599This happens because the pattern match is in list context (since it
600is being assigned to the empty list), and will therefore return a list
601of all matching parts of the string. The list assignment in scalar
602context will translate that into the number of elements (here, the
603number of times the pattern matched) and assign that to $count. Note
604that simply using
605
84f709e7 606 $count = $string =~ /\d+/g;
ab1f959b
PN
607
608would not have worked, since a pattern match in scalar context will
609only return true or false, rather than a count of matches.
610
611The final element of a list assignment may be an array or a hash:
a0d0e21e 612
84f709e7 613 ($a, $b, @rest) = split;
5a964f20 614 my($a, $b, %rest) = @_;
a0d0e21e 615
4633a7c4 616You can actually put an array or hash anywhere in the list, but the first one
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617in the list will soak up all the values, and anything after it will become
618undefined. This may be useful in a my() or local().
a0d0e21e 619
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620A hash can be initialized using a literal list holding pairs of
621items to be interpreted as a key and a value:
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LW
622
623 # same as map assignment above
84f709e7 624 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
a0d0e21e 625
d55a8828 626While literal lists and named arrays are often interchangeable, that's
4633a7c4
LW
627not the case for hashes. Just because you can subscript a list value like
628a normal array does not mean that you can subscript a list value as a
629hash. Likewise, hashes included as parts of other lists (including
630parameters lists and return lists from functions) always flatten out into
631key/value pairs. That's why it's good to use references sometimes.
a0d0e21e 632
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633It is often more readable to use the C<< => >> operator between key/value
634pairs. The C<< => >> operator is mostly just a more visually distinctive
b88cefa9 635synonym for a comma, but it also arranges for its left-hand operand to be
ac036724 636interpreted as a string if it's a bareword that would be a legal simple
637identifier. C<< => >> doesn't quote compound identifiers, that contain
638double colons. This makes it nice for initializing hashes:
a0d0e21e 639
84f709e7 640 %map = (
7e3b091d
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641 red => 0x00f,
642 blue => 0x0f0,
643 green => 0xf00,
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LW
644 );
645
646or for initializing hash references to be used as records:
647
84f709e7 648 $rec = {
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649 witch => 'Mable the Merciless',
650 cat => 'Fluffy the Ferocious',
651 date => '10/31/1776',
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LW
652 };
653
654or for using call-by-named-parameter to complicated functions:
655
84f709e7 656 $field = $query->radio_group(
7e3b091d 657 name => 'group_name',
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LW
658 values => ['eenie','meenie','minie'],
659 default => 'meenie',
660 linebreak => 'true',
84f709e7 661 labels => \%labels
4633a7c4 662 );
cb1a09d0
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663
664Note that just because a hash is initialized in that order doesn't
665mean that it comes out in that order. See L<perlfunc/sort> for examples
666of how to arrange for an output ordering.
667
692ef166
SF
668=head2 Subscripts
669
fa11829f 670An array is subscripted by specifying a dollar sign (C<$>), then the
692ef166
SF
671name of the array (without the leading C<@>), then the subscript inside
672square brackets. For example:
673
674 @myarray = (5, 50, 500, 5000);
2adc35dd 675 print "The Third Element is", $myarray[2], "\n";
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676
677The array indices start with 0. A negative subscript retrieves its
678value from the end. In our example, C<$myarray[-1]> would have been
6795000, and C<$myarray[-2]> would have been 500.
680
681Hash subscripts are similar, only instead of square brackets curly brackets
682are used. For example:
683
684 %scientists =
685 (
686 "Newton" => "Isaac",
687 "Einstein" => "Albert",
688 "Darwin" => "Charles",
689 "Feynman" => "Richard",
690 );
691
692 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
693
d55a8828 694=head2 Slices
d74e8afc 695X<slice> X<array, slice> X<hash, slice>
d55a8828 696
56d7751a
GS
697A common way to access an array or a hash is one scalar element at a
698time. You can also subscript a list to get a single element from it.
d55a8828 699
7e3b091d
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700 $whoami = $ENV{"USER"}; # one element from the hash
701 $parent = $ISA[0]; # one element from the array
702 $dir = (getpwnam("daemon"))[7]; # likewise, but with list
d55a8828
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703
704A slice accesses several elements of a list, an array, or a hash
56d7751a
GS
705simultaneously using a list of subscripts. It's more convenient
706than writing out the individual elements as a list of separate
d55a8828
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707scalar values.
708
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709 ($him, $her) = @folks[0,-1]; # array slice
710 @them = @folks[0 .. 3]; # array slice
711 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
712 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
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713
714Since you can assign to a list of variables, you can also assign to
715an array or hash slice.
716
84f709e7 717 @days[3..5] = qw/Wed Thu Fri/;
d55a8828 718 @colors{'red','blue','green'}
7e3b091d 719 = (0xff0000, 0x0000ff, 0x00ff00);
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720 @folks[0, -1] = @folks[-1, 0];
721
722The previous assignments are exactly equivalent to
723
84f709e7
JH
724 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
725 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
7e3b091d 726 = (0xff0000, 0x0000ff, 0x00ff00);
88fd19e3 727 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
d55a8828
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728
729Since changing a slice changes the original array or hash that it's
56d7751a
GS
730slicing, a C<foreach> construct will alter some--or even all--of the
731values of the array or hash.
d55a8828
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732
733 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
734
00cb5da1 735 foreach (@hash{qw[key1 key2]}) {
7e3b091d
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736 s/^\s+//; # trim leading whitespace
737 s/\s+$//; # trim trailing whitespace
738 s/(\w+)/\u\L$1/g; # "titlecase" words
d55a8828
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739 }
740
08cd8952
GS
741A slice of an empty list is still an empty list. Thus:
742
84f709e7
JH
743 @a = ()[1,0]; # @a has no elements
744 @b = (@a)[0,1]; # @b has no elements
745 @c = (0,1)[2,3]; # @c has no elements
56d7751a
GS
746
747But:
748
84f709e7
JH
749 @a = (1)[1,0]; # @a has two elements
750 @b = (1,undef)[1,0,2]; # @b has three elements
08cd8952 751
19799a22
GS
752This makes it easy to write loops that terminate when a null list
753is returned:
d55a8828 754
84f709e7 755 while ( ($home, $user) = (getpwent)[7,0]) {
7e3b091d 756 printf "%-8s %s\n", $user, $home;
d55a8828
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757 }
758
759As noted earlier in this document, the scalar sense of list assignment
760is the number of elements on the right-hand side of the assignment.
19799a22 761The null list contains no elements, so when the password file is
d55a8828
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762exhausted, the result is 0, not 2.
763
ad1de9c6
ML
764Slices in scalar context return the last item of the slice.
765
766 @a = qw/first second third/;
767 %h = (first => 'A', second => 'B');
768 $t = @a[0, 1]; # $t is now 'second'
0de10106 769 $u = @h{'first', 'second'}; # $u is now 'B'
ad1de9c6 770
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771If you're confused about why you use an '@' there on a hash slice
772instead of a '%', think of it like this. The type of bracket (square
773or curly) governs whether it's an array or a hash being looked at.
774On the other hand, the leading symbol ('$' or '@') on the array or
775hash indicates whether you are getting back a singular value (a
776scalar) or a plural one (a list).
777
5f05dabc 778=head2 Typeglobs and Filehandles
d74e8afc 779X<typeglob> X<filehandle> X<*>
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780
781Perl uses an internal type called a I<typeglob> to hold an entire
782symbol table entry. The type prefix of a typeglob is a C<*>, because
54310121 783it represents all types. This used to be the preferred way to
cb1a09d0 784pass arrays and hashes by reference into a function, but now that
5a964f20
TC
785we have real references, this is seldom needed.
786
787The main use of typeglobs in modern Perl is create symbol table aliases.
788This assignment:
789
790 *this = *that;
791
792makes $this an alias for $that, @this an alias for @that, %this an alias
793for %that, &this an alias for &that, etc. Much safer is to use a reference.
794This:
5f05dabc 795
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796 local *Here::blue = \$There::green;
797
798temporarily makes $Here::blue an alias for $There::green, but doesn't
799make @Here::blue an alias for @There::green, or %Here::blue an alias for
800%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
801of this. Strange though this may seem, this is the basis for the whole
84f709e7 802module import/export system.
5a964f20 803
d55a8828 804Another use for typeglobs is to pass filehandles into a function or
5a964f20
TC
805to create new filehandles. If you need to use a typeglob to save away
806a filehandle, do it this way:
5f05dabc 807
84f709e7 808 $fh = *STDOUT;
5f05dabc 809
810or perhaps as a real reference, like this:
811
84f709e7 812 $fh = \*STDOUT;
5f05dabc 813
5a964f20
TC
814See L<perlsub> for examples of using these as indirect filehandles
815in functions.
816
817Typeglobs are also a way to create a local filehandle using the local()
818operator. These last until their block is exited, but may be passed back.
819For example:
5f05dabc 820
821 sub newopen {
7e3b091d
DA
822 my $path = shift;
823 local *FH; # not my!
824 open (FH, $path) or return undef;
825 return *FH;
5f05dabc 826 }
84f709e7 827 $fh = newopen('/etc/passwd');
5f05dabc 828
d55a8828 829Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
5a964f20
TC
830for filehandle manipulations, although they're still needed to pass brand
831new file and directory handles into or out of functions. That's because
d55a8828
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832C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
833In other words, C<*FH> must be used to create new symbol table entries;
834C<*foo{THING}> cannot. When in doubt, use C<*FH>.
835
36392fcf
GS
836All functions that are capable of creating filehandles (open(),
837opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
838automatically create an anonymous filehandle if the handle passed to
839them is an uninitialized scalar variable. This allows the constructs
840such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
841create filehandles that will conveniently be closed automatically when
842the scope ends, provided there are no other references to them. This
843largely eliminates the need for typeglobs when opening filehandles
844that must be passed around, as in the following example:
845
846 sub myopen {
84f709e7 847 open my $fh, "@_"
7e3b091d
DA
848 or die "Can't open '@_': $!";
849 return $fh;
36392fcf
GS
850 }
851
852 {
853 my $f = myopen("</etc/motd");
7e3b091d
DA
854 print <$f>;
855 # $f implicitly closed here
36392fcf
GS
856 }
857
b92795fe
AMS
858Note that if an initialized scalar variable is used instead the
859result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
860to C<open( *{'zzz'}, ...)>.
d83fe814
AT
861C<use strict 'refs'> forbids such practice.
862
d55a8828
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863Another way to create anonymous filehandles is with the Symbol
864module or with the IO::Handle module and its ilk. These modules
865have the advantage of not hiding different types of the same name
866during the local(). See the bottom of L<perlfunc/open()> for an
867example.
868
869=head1 SEE ALSO
870
871See L<perlvar> for a description of Perl's built-in variables and
872a discussion of legal variable names. See L<perlref>, L<perlsub>,
873and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
874the C<*foo{THING}> syntax.