=head1 DESCRIPTION
=head2 Variable names
+X<variable, name> X<variable name> X<data type> X<type>
-Perl has three data structures: scalars, arrays of scalars, and
-associative arrays of scalars, known as "hashes". Normal arrays are
-indexed by number, starting with 0. (Negative subscripts count from
-the end.) Hash arrays are indexed by string.
+Perl has three built-in data types: scalars, arrays of scalars, and
+associative arrays of scalars, known as "hashes". A scalar is a
+single string (of any size, limited only by the available memory),
+number, or a reference to something (which will be discussed
+in L<perlref>). Normal arrays are ordered lists of scalars indexed
+by number, starting with 0. Hashes are unordered collections of scalar
+values indexed by their associated string key.
-Values are usually referred to by name (or through a named reference).
+Values are usually referred to by name, or through a named reference.
The first character of the name tells you to what sort of data
structure it refers. The rest of the name tells you the particular
-value to which it refers. Most often, it consists of a single
-I<identifier>, that is, a string beginning with a letter or underscore,
-and containing letters, underscores, and digits. In some cases, it
-may be a chain of identifiers, separated by C<::> (or by C<'>, but
-that's deprecated); all but the last are interpreted as names of
-packages, to locate the namespace in which to look
-up the final identifier (see L<perlmod/Packages> for details).
-It's possible to substitute for a simple identifier an expression
-that produces a reference to the value at runtime; this is
-described in more detail below, and in L<perlref>.
-
-There are also special variables whose names don't follow these
-rules, so that they don't accidentally collide with one of your
-normal variables. Strings that match parenthesized parts of a
-regular expression are saved under names containing only digits after
-the C<$> (see L<perlop> and L<perlre>). In addition, several special
-variables that provide windows into the inner working of Perl have names
-containing punctuation characters (see L<perlvar>).
-
-Scalar values are always named with '$', even when referring to a scalar
-that is part of an array. It works like the English word "the". Thus
-we have:
+value to which it refers. Usually this name is a single I<identifier>,
+that is, a string beginning with a letter or underscore, and
+containing letters, underscores, and digits. In some cases, it may
+be a chain of identifiers, separated by C<::> (or by the slightly
+archaic C<'>); all but the last are interpreted as names of packages,
+to locate the namespace in which to look up the final identifier
+(see L<perlmod/Packages> for details). For a more in-depth discussion
+on identifiers, see L<Identifier parsing>. It's possible to
+substitute for a simple identifier, an expression that produces a reference
+to the value at runtime. This is described in more detail below
+and in L<perlref>.
+X<identifier>
+
+Perl also has its own built-in variables whose names don't follow
+these rules. They have strange names so they don't accidentally
+collide with one of your normal variables. Strings that match
+parenthesized parts of a regular expression are saved under names
+containing only digits after the C<$> (see L<perlop> and L<perlre>).
+In addition, several special variables that provide windows into
+the inner working of Perl have names containing punctuation characters
+and control characters. These are documented in L<perlvar>.
+X<variable, built-in>
+
+Scalar values are always named with '$', even when referring to a
+scalar that is part of an array or a hash. The '$' symbol works
+semantically like the English word "the" in that it indicates a
+single value is expected.
+X<scalar>
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
-but entire arrays or array slices are denoted by '@', which works much like
-the word "these" or "those":
+Entire arrays (and slices of arrays and hashes) are denoted by '@',
+which works much as the word "these" or "those" does in English,
+in that it indicates multiple values are expected.
+X<array>
@days # ($days[0], $days[1],... $days[n])
- @days[3,4,5] # same as @days[3..5]
+ @days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
-and entire hashes are denoted by '%':
+Entire hashes are denoted by '%':
+X<hash>
%days # (key1, val1, key2, val2 ...)
-In addition, subroutines are named with an initial '&', though this is
-optional when it's otherwise unambiguous (just as "do" is often
-redundant in English). Symbol table entries can be named with an
-initial '*', but you don't really care about that yet.
-
-Every variable type has its own namespace. You can, without fear of
-conflict, use the same name for a scalar variable, an array, or a hash
-(or, for that matter, a filehandle, a subroutine name, or a label).
-This means that $foo and @foo are two different variables. It also
-means that C<$foo[1]> is a part of @foo, not a part of $foo. This may
-seem a bit weird, but that's okay, because it is weird.
-
-Because variable and array references always start with '$', '@', or '%',
-the "reserved" words aren't in fact reserved with respect to variable
-names. (They ARE reserved with respect to labels and filehandles,
-however, which don't have an initial special character. You can't have
-a filehandle named "log", for instance. Hint: you could say
-C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
-filehandles also improves readability and protects you from conflict
-with future reserved words.) Case I<IS> significant--"FOO", "Foo", and
-"foo" are all different names. Names that start with a letter or
-underscore may also contain digits and underscores.
+In addition, subroutines are named with an initial '&', though this
+is optional when unambiguous, just as the word "do" is often redundant
+in English. Symbol table entries can be named with an initial '*',
+but you don't really care about that yet (if ever :-).
+
+Every variable type has its own namespace, as do several
+non-variable identifiers. This means that you can, without fear
+of conflict, use the same name for a scalar variable, an array, or
+a hash--or, for that matter, for a filehandle, a directory handle, a
+subroutine name, a format name, or a label. This means that $foo
+and @foo are two different variables. It also means that C<$foo[1]>
+is a part of @foo, not a part of $foo. This may seem a bit weird,
+but that's okay, because it is weird.
+X<namespace>
+
+Because variable references always start with '$', '@', or '%', the
+"reserved" words aren't in fact reserved with respect to variable
+names. They I<are> reserved with respect to labels and filehandles,
+however, which don't have an initial special character. You can't
+have a filehandle named "log", for instance. Hint: you could say
+C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
+uppercase filehandles also improves readability and protects you
+from conflict with future reserved words. Case I<is> significant--"FOO",
+"Foo", and "foo" are all different names. Names that start with a
+letter or underscore may also contain digits and underscores.
+X<identifier, case sensitivity>
+X<case>
It is possible to replace such an alphanumeric name with an expression
-that returns a reference to an object of that type. For a description
+that returns a reference to the appropriate type. For a description
of this, see L<perlref>.
Names that start with a digit may contain only more digits. Names
-that do not start with a letter, underscore, or digit are limited to
-one character, e.g., C<$%> or C<$$>. (Most of these one character names
-have a predefined significance to Perl. For instance, C<$$> is the
-current process id.)
+that do not start with a letter, underscore, digit or a caret (i.e.
+a control character) are limited to one character, e.g., C<$%> or
+C<$$>. (Most of these one character names have a predefined
+significance to Perl. For instance, C<$$> is the current process
+id.)
+
+=head2 Identifier parsing
+X<identifiers>
+
+Up until Perl 5.18, the actual rules of what a valid identifier
+was were a bit fuzzy. However, in general, anything defined here should
+work on previous versions of Perl, while the opposite -- edge cases
+that work in previous versions, but aren't defined here -- probably
+won't work on newer versions.
+As an important side note, please note that the following only applies
+to bareword identifiers as found in Perl source code, not identifiers
+introduced through symbolic references, which have much fewer
+restrictions.
+If working under the effect of the C<use utf8;> pragma, the following
+rules apply:
+
+ / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
+ (?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
+
+That is, a "start" character followed by any number of "continue"
+characters. Perl requires every character in an identifier to also
+match C<\w> (this prevents some problematic cases); and Perl
+additionally accepts identfier names beginning with an underscore.
+
+If not under C<use utf8>, the source is treated as ASCII + 128 extra
+controls, and identifiers should match
+
+ / (?aa) (?!\d) \w+ /x
+
+That is, any word character in the ASCII range, as long as the first
+character is not a digit.
+
+There are two package separators in Perl: A double colon (C<::>) and a single
+quote (C<'>). Normal identifiers can start or end with a double colon, and
+can contain several parts delimited by double colons.
+Single quotes have similar rules, but with the exception that they are not
+legal at the end of an identifier: That is, C<$'foo> and C<$foo'bar> are
+legal, but C<$foo'bar'> is not.
+
+Additionally, if the identifier is preceded by a sigil --
+that is, if the identifier is part of a variable name -- it
+may optionally be enclosed in braces.
+
+While you can mix double colons with singles quotes, the quotes must come
+after the colons: C<$::::'foo> and C<$foo::'bar> are legal, but C<$::'::foo>
+and C<$foo'::bar> are not.
+
+Put together, a grammar to match a basic identifier becomes
+
+ /
+ (?(DEFINE)
+ (?<variable>
+ (?&sigil)
+ (?:
+ (?&normal_identifier)
+ | \{ \s* (?&normal_identifier) \s* \}
+ )
+ )
+ (?<normal_identifier>
+ (?: :: )* '?
+ (?&basic_identifier)
+ (?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
+ (?: :: )*
+ )
+ (?<basic_identifier>
+ # is use utf8 on?
+ (?(?{ (caller(0))[8] & $utf8::hint_bits })
+ (?&Perl_XIDS) (?&Perl_XIDC)*
+ | (?aa) (?!\d) \w+
+ )
+ )
+ (?<sigil> [&*\$\@\%])
+ (?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
+ (?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
+ )
+ /x
+
+Meanwhile, special identifiers don't follow the above rules; For the most
+part, all of the identifiers in this category have a special meaning given
+by Perl. Because they have special parsing rules, these generally can't be
+fully-qualified. They come in four forms:
+
+=over
+
+=item A sigil, followed solely by digits matching \p{POSIX_Digit}, like C<$0>,
+C<$1>, or C<$10000>.
+
+=item A sigil, followed by either a caret and a single POSIX uppercase letter,
+like C<$^V> or C<$^W>, or a sigil followed by a literal control character
+matching the C<\p{POSIX_Cntrl}> property.
+Due to a historical oddity, if not
+running under C<use utf8>, the 128 extra controls in the C<[0x80-0xff]> range
+may also be used in length one variables. The use of a literal control
+character is deprecated. Support for this form will be removed in a future
+version of perl.
+
+=item Similar to the above, a sigil, followed by bareword text in brackets,
+where the first character is either a caret followed by an uppercase letter,
+or a literal control, like C<${^GLOBAL_PHASE}> or C<${\7LOBAL_PHASE}>. The use
+of a literal control character is deprecated. Support for this form will be
+removed in a future version of perl.
+
+=item A sigil followed by a single character matching the C<\p{POSIX_Punct}>
+property, like C<$!> or C<%+>.
+
+=back
+
+Note that as of Perl 5.20, literal control characters in variable names
+are deprecated.
=head2 Context
+X<context> X<scalar context> X<list context>
The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
-There are two major contexts: scalar and list. Certain operations
+There are two major contexts: list and scalar. Certain operations
return list values in contexts wanting a list, and scalar values
-otherwise. (If this is true of an operation it will be mentioned in
-the documentation for that operation.) In other words, Perl overloads
+otherwise. If this is true of an operation it will be mentioned in
+the documentation for that operation. In other words, Perl overloads
certain operations based on whether the expected return value is
-singular or plural. (Some words in English work this way, like "fish"
-and "sheep".)
+singular or plural. Some words in English work this way, like "fish"
+and "sheep".
In a reciprocal fashion, an operation provides either a scalar or a
list context to each of its arguments. For example, if you say
int( <STDIN> )
-the integer operation provides a scalar context for the E<lt>STDINE<gt>
+the integer operation provides scalar context for the <>
operator, which responds by reading one line from STDIN and passing it
back to the integer operation, which will then find the integer value
of that line and return that. If, on the other hand, you say
sort( <STDIN> )
-then the sort operation provides a list context for E<lt>STDINE<gt>, which
+then the sort operation provides list context for <>, which
will proceed to read every line available up to the end of file, and
pass that list of lines back to the sort routine, which will then
sort those lines and return them as a list to whatever the context
of the sort was.
-Assignment is a little bit special in that it uses its left argument to
-determine the context for the right argument. Assignment to a scalar
-evaluates the righthand side in a scalar context, while assignment to
-an array or array slice evaluates the righthand side in a list
-context. Assignment to a list also evaluates the righthand side in a
-list context.
-
-User defined subroutines may choose to care whether they are being
-called in a scalar or list context, but most subroutines do not
-need to care, because scalars are automatically interpolated into
-lists. See L<perlfunc/wantarray>.
+Assignment is a little bit special in that it uses its left argument
+to determine the context for the right argument. Assignment to a
+scalar evaluates the right-hand side in scalar context, while
+assignment to an array or hash evaluates the righthand side in list
+context. Assignment to a list (or slice, which is just a list
+anyway) also evaluates the right-hand side in list context.
+
+When you use the C<use warnings> pragma or Perl's B<-w> command-line
+option, you may see warnings
+about useless uses of constants or functions in "void context".
+Void context just means the value has been discarded, such as a
+statement containing only C<"fred";> or C<getpwuid(0);>. It still
+counts as scalar context for functions that care whether or not
+they're being called in list context.
+
+User-defined subroutines may choose to care whether they are being
+called in a void, scalar, or list context. Most subroutines do not
+need to bother, though. That's because both scalars and lists are
+automatically interpolated into lists. See L<perlfunc/wantarray>
+for how you would dynamically discern your function's calling
+context.
=head2 Scalar values
-
-All data in Perl is a scalar or an array of scalars or a hash of scalars.
-Scalar variables may contain various kinds of singular data, such as
-numbers, strings, and references. In general, conversion from one form to
-another is transparent. (A scalar may not contain multiple values, but
-may contain a reference to an array or hash containing multiple values.)
-Because of the automatic conversion of scalars, operations, and functions
-that return scalars don't need to care (and, in fact, can't care) whether
-the context is looking for a string or a number.
-
-Scalars aren't necessarily one thing or another. There's no place to
-declare a scalar variable to be of type "string", or of type "number", or
-type "filehandle", or anything else. Perl is a contextually polymorphic
-language whose scalars can be strings, numbers, or references (which
-includes objects). While strings and numbers are considered pretty
-much the same thing for nearly all purposes, references are strongly-typed
-uncastable pointers with builtin reference-counting and destructor
-invocation.
-
-A scalar value is interpreted as TRUE in the Boolean sense if it is not
-the null string or the number 0 (or its string equivalent, "0"). The
-Boolean context is just a special kind of scalar context.
-
-There are actually two varieties of null scalars: defined and
-undefined. Undefined null scalars are returned when there is no real
-value for something, such as when there was an error, or at end of
-file, or when you refer to an uninitialized variable or element of an
-array. An undefined null scalar may become defined the first time you
-use it as if it were defined, but prior to that you can use the
-defined() operator to determine whether the value is defined or not.
-
-To find out whether a given string is a valid nonzero number, it's usually
-enough to test it against both numeric 0 and also lexical "0" (although
-this will cause B<-w> noises). That's because strings that aren't
-numbers count as 0, just as they do in B<awk>:
+X<scalar> X<number> X<string> X<reference>
+
+All data in Perl is a scalar, an array of scalars, or a hash of
+scalars. A scalar may contain one single value in any of three
+different flavors: a number, a string, or a reference. In general,
+conversion from one form to another is transparent. Although a
+scalar may not directly hold multiple values, it may contain a
+reference to an array or hash which in turn contains multiple values.
+
+Scalars aren't necessarily one thing or another. There's no place
+to declare a scalar variable to be of type "string", type "number",
+type "reference", or anything else. Because of the automatic
+conversion of scalars, operations that return scalars don't need
+to care (and in fact, cannot care) whether their caller is looking
+for a string, a number, or a reference. Perl is a contextually
+polymorphic language whose scalars can be strings, numbers, or
+references (which includes objects). Although strings and numbers
+are considered pretty much the same thing for nearly all purposes,
+references are strongly-typed, uncastable pointers with builtin
+reference-counting and destructor invocation.
+
+A scalar value is interpreted as FALSE in the Boolean sense
+if it is undefined, the null string or the number 0 (or its
+string equivalent, "0"), and TRUE if it is anything else. The
+Boolean context is just a special kind of scalar context where no
+conversion to a string or a number is ever performed.
+X<boolean> X<bool> X<true> X<false> X<truth>
+
+There are actually two varieties of null strings (sometimes referred
+to as "empty" strings), a defined one and an undefined one. The
+defined version is just a string of length zero, such as C<"">.
+The undefined version is the value that indicates that there is
+no real value for something, such as when there was an error, or
+at end of file, or when you refer to an uninitialized variable or
+element of an array or hash. Although in early versions of Perl,
+an undefined scalar could become defined when first used in a
+place expecting a defined value, this no longer happens except for
+rare cases of autovivification as explained in L<perlref>. You can
+use the defined() operator to determine whether a scalar value is
+defined (this has no meaning on arrays or hashes), and the undef()
+operator to produce an undefined value.
+X<defined> X<undefined> X<undef> X<null> X<string, null>
+
+To find out whether a given string is a valid non-zero number, it's
+sometimes enough to test it against both numeric 0 and also lexical
+"0" (although this will cause noises if warnings are on). That's
+because strings that aren't numbers count as 0, just as they do in B<awk>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
-That's usually preferable because otherwise you won't treat IEEE notations
-like C<NaN> or C<Infinity> properly. At other times you might prefer to
-use the POSIX::strtod function or a regular expression to check whether
-data is numeric. See L<perlre> for details on regular expressions.
+That method may be best because otherwise you won't treat IEEE
+notations like C<NaN> or C<Infinity> properly. At other times, you
+might prefer to determine whether string data can be used numerically
+by calling the POSIX::strtod() function or by inspecting your string
+with a regular expression (as documented in L<perlre>).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # rejects -3
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
-The length of an array is a scalar value. You may find the length of
-array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not
-the length of the array, it's the subscript of the last element, because
-there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the
-length of the array. Shortening an array by this method destroys
-intervening values. Lengthening an array that was previously shortened
-I<NO LONGER> recovers the values that were in those elements. (It used to
-in Perl 4, but we had to break this to make sure destructors were
-called when expected.) You can also gain some miniscule measure of efficiency by
-pre-extending an array that is going to get big. (You can also extend
-an array by assigning to an element that is off the end of the array.)
-You can truncate an array down to nothing by assigning the null list ()
-to it. The following are equivalent:
+The length of an array is a scalar value. You may find the length
+of array @days by evaluating C<$#days>, as in B<csh>. However, this
+isn't the length of the array; it's the subscript of the last element,
+which is a different value since there is ordinarily a 0th element.
+Assigning to C<$#days> actually changes the length of the array.
+Shortening an array this way destroys intervening values. Lengthening
+an array that was previously shortened does not recover values
+that were in those elements.
+X<$#> X<array, length>
+
+You can also gain some minuscule measure of efficiency by pre-extending
+an array that is going to get big. You can also extend an array
+by assigning to an element that is off the end of the array. You
+can truncate an array down to nothing by assigning the null list
+() to it. The following are equivalent:
@whatever = ();
$#whatever = -1;
-If you evaluate a named array in a scalar context, it returns the length of
-the array. (Note that this is not true of lists, which return the
-last value, like the C comma operator, nor of built-in functions, which return
-whatever they feel like returning.) The following is always true:
-
- scalar(@whatever) == $#whatever - $[ + 1;
-
-Version 5 of Perl changed the semantics of C<$[>: files that don't set
-the value of C<$[> no longer need to worry about whether another
-file changed its value. (In other words, use of C<$[> is deprecated.)
-So in general you can assume that
+If you evaluate an array in scalar context, it returns the length
+of the array. (Note that this is not true of lists, which return
+the last value, like the C comma operator, nor of built-in functions,
+which return whatever they feel like returning.) The following is
+always true:
+X<array, length>
scalar(@whatever) == $#whatever + 1;
-Some programmers choose to use an explicit conversion so nothing's
-left to doubt:
+Some programmers choose to use an explicit conversion so as to
+leave nothing to doubt:
$element_count = scalar(@whatever);
-If you evaluate a hash in a scalar context, it returns a value that is
-true if and only if the hash contains any key/value pairs. (If there
-are any key/value pairs, the value returned is a string consisting of
-the number of used buckets and the number of allocated buckets, separated
-by a slash. This is pretty much useful only to find out whether Perl's
-(compiled in) hashing algorithm is performing poorly on your data set.
-For example, you stick 10,000 things in a hash, but evaluating %HASH in
-scalar context reveals "1/16", which means only one out of sixteen buckets
-has been touched, and presumably contains all 10,000 of your items. This
-isn't supposed to happen.)
+If you evaluate a hash in scalar context, it returns false if the
+hash is empty. If there are any key/value pairs, it returns true;
+more precisely, the value returned is a string consisting of the
+number of used buckets and the number of allocated buckets, separated
+by a slash. This is pretty much useful only to find out whether
+Perl's internal hashing algorithm is performing poorly on your data
+set. For example, you stick 10,000 things in a hash, but evaluating
+%HASH in scalar context reveals C<"1/16">, which means only one out
+of sixteen buckets has been touched, and presumably contains all
+10,000 of your items. This isn't supposed to happen. If a tied hash
+is evaluated in scalar context, the C<SCALAR> method is called (with a
+fallback to C<FIRSTKEY>).
+X<hash, scalar context> X<hash, bucket> X<bucket>
You can preallocate space for a hash by assigning to the keys() function.
-This rounds up the allocated bucked to the next power of two:
+This rounds up the allocated buckets to the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
=head2 Scalar value constructors
+X<scalar, literal> X<scalar, constant>
-Numeric literals are specified in any of the customary floating point or
+Numeric literals are specified in any of the following floating point or
integer formats:
- 12345
- 12345.67
- .23E-10
- 0xffff # hex
- 0377 # octal
- 4_294_967_296 # underline for legibility
+ 12345
+ 12345.67
+ .23E-10 # a very small number
+ 3.14_15_92 # a very important number
+ 4_294_967_296 # underscore for legibility
+ 0xff # hex
+ 0xdead_beef # more hex
+ 0377 # octal (only numbers, begins with 0)
+ 0b011011 # binary
+ 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
+
+You are allowed to use underscores (underbars) in numeric literals
+between digits for legibility (but not multiple underscores in a row:
+C<23__500> is not legal; C<23_500> is).
+You could, for example, group binary
+digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
+or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
+X<number, literal>
String literals are usually delimited by either single or double
-quotes. They work much like shell quotes: double-quoted string
-literals are subject to backslash and variable substitution;
-single-quoted strings are not (except for "C<\'>" and "C<\\>").
-The usual Unix backslash rules apply for making characters such as
-newline, tab, etc., as well as some more exotic forms. See
-L<perlop/"Quote and Quotelike Operators"> for a list.
-
-Octal or hex representations in string literals (e.g. '0xffff') are not
-automatically converted to their integer representation. The hex() and
-oct() functions make these conversions for you. See L<perlfunc/hex> and
-L<perlfunc/oct> for more details.
+quotes. They work much like quotes in the standard Unix shells:
+double-quoted string literals are subject to backslash and variable
+substitution; single-quoted strings are not (except for C<\'> and
+C<\\>). The usual C-style backslash rules apply for making
+characters such as newline, tab, etc., as well as some more exotic
+forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
+X<string, literal>
+
+Hexadecimal, octal, or binary, representations in string literals
+(e.g. '0xff') are not automatically converted to their integer
+representation. The hex() and oct() functions make these conversions
+for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
+
+Hexadecimal floating point can start just like a hexadecimal literal,
+and it can be followed by an optional fractional hexadecimal part,
+but it must be followed by C<p>, an optional sign, and a power of two.
+The format is useful for accurately presenting floating point values,
+avoiding conversions to or from decimal floating point, and therefore
+avoiding possible loss in precision. Notice that while most current
+platforms use the 64-bit IEEE 754 floating point, not all do.
You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin. This is nice, but if you forget
your trailing quote, the error will not be reported until Perl finds
another line containing the quote character, which may be much further
on in the script. Variable substitution inside strings is limited to
-scalar variables, arrays, and array slices. (In other words,
+scalar variables, arrays, and array or hash slices. (In other words,
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
-
- $Price = '$100'; # not interpreted
- print "The price is $Price.\n"; # interpreted
-
-As in some shells, you can put curly brackets around the name to
-delimit it from following alphanumerics. In fact, an identifier
-within such curlies is forced to be a string, as is any single
-identifier within a hash subscript. Our earlier example,
-
- $days{'Feb'}
-
-can be written as
-
- $days{Feb}
-
-and the quotes will be assumed automatically. But anything more complicated
-in the subscript will be interpreted as an expression.
-
-Note that a
-single-quoted string must be separated from a preceding word by a
-space, because single quote is a valid (though deprecated) character in
-a variable name (see L<perlmod/Packages>).
-
-Three special literals are __FILE__, __LINE__, and __PACKAGE__, which
+X<interpolation>
+
+ $Price = '$100'; # not interpolated
+ print "The price is $Price.\n"; # interpolated
+
+There is no double interpolation in Perl, so the C<$100> is left as is.
+
+By default floating point numbers substituted inside strings use the
+dot (".") as the decimal separator. If C<use locale> is in effect,
+and POSIX::setlocale() has been called, the character used for the
+decimal separator is affected by the LC_NUMERIC locale.
+See L<perllocale> and L<POSIX>.
+
+As in some shells, you can enclose the variable name in braces to
+disambiguate it from following alphanumerics (and underscores).
+You must also do
+this when interpolating a variable into a string to separate the
+variable name from a following double-colon or an apostrophe, since
+these would be otherwise treated as a package separator:
+X<interpolation>
+
+ $who = "Larry";
+ print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
+ print "We use ${who}speak when ${who}'s here.\n";
+
+Without the braces, Perl would have looked for a $whospeak, a
+C<$who::0>, and a C<$who's> variable. The last two would be the
+$0 and the $s variables in the (presumably) non-existent package
+C<who>.
+
+In fact, a simple identifier within such curlies is forced to be
+a string, and likewise within a hash subscript. Neither need
+quoting. Our earlier example, C<$days{'Feb'}> can be written as
+C<$days{Feb}> and the quotes will be assumed automatically. But
+anything more complicated in the subscript will be interpreted as an
+expression. This means for example that C<$version{2.0}++> is
+equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
+
+=head3 Version Strings
+X<version string> X<vstring> X<v-string>
+
+A literal of the form C<v1.20.300.4000> is parsed as a string composed
+of characters with the specified ordinals. This form, known as
+v-strings, provides an alternative, more readable way to construct
+strings, rather than use the somewhat less readable interpolation form
+C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
+Unicode strings, and for comparing version "numbers" using the string
+comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
+more dots in the literal, the leading C<v> may be omitted.
+
+ print v9786; # prints SMILEY, "\x{263a}"
+ print v102.111.111; # prints "foo"
+ print 102.111.111; # same
+
+Such literals are accepted by both C<require> and C<use> for
+doing a version check. Note that using the v-strings for IPv4
+addresses is not portable unless you also use the
+inet_aton()/inet_ntoa() routines of the Socket package.
+
+Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
+are not v-strings before the C<< => >> operator (which is usually used
+to separate a hash key from a hash value); instead they are interpreted
+as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
+Perl 5.8.0, but that caused more confusion and breakage than good.
+Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
+be v-strings always.
+
+=head3 Special Literals
+X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
+X<end> X<data> X<^D> X<^Z>
+
+The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package name at that
-point in your program. They may be used only as separate tokens; they
+point in your program. __SUB__ gives a reference to the current
+subroutine. They may be used only as separate tokens; they
will not be interpolated into strings. If there is no current package
-(due to an empty C<package;> directive), __PACKAGE__ is the undefined value.
-
-The tokens __END__ and __DATA__ may be used to indicate the logical end
-of the script before the actual end of file. Any following text is
-ignored, but may be read via a DATA filehandle: main::DATA for __END__,
-or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__.
-The two control characters ^D and ^Z are synonyms for __END__ (or
-__DATA__ in a module). See L<SelfLoader> for more description of
-__DATA__, and an example of its use. Note that you cannot read from the
-DATA filehandle in a BEGIN block: the BEGIN block is executed as soon as
-it is seen (during compilation), at which point the corresponding
+(due to an empty C<package;> directive), __PACKAGE__ is the undefined
+value. (But the empty C<package;> is no longer supported, as of version
+5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
+is only available in 5.16 or higher, and only with a C<use v5.16> or
+C<use feature "current_sub"> declaration.
+X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
+X<line> X<file> X<package>
+
+The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
+may be used to indicate the logical end of the script before the actual
+end of file. Any following text is ignored.
+
+Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
+where C<PACKNAME> is the package that was current when the __DATA__
+token was encountered. The filehandle is left open pointing to the
+line after __DATA__. The program should C<close DATA> when it is done
+reading from it. (Leaving it open leaks filehandles if the module is
+reloaded for any reason, so it's a safer practice to close it.) For
+compatibility with older scripts written before __DATA__ was
+introduced, __END__ behaves like __DATA__ in the top level script (but
+not in files loaded with C<require> or C<do>) and leaves the remaining
+contents of the file accessible via C<main::DATA>.
+
+See L<SelfLoader> for more description of __DATA__, and
+an example of its use. Note that you cannot read from the DATA
+filehandle in a BEGIN block: the BEGIN block is executed as soon
+as it is seen (during compilation), at which point the corresponding
__DATA__ (or __END__) token has not yet been seen.
+=head3 Barewords
+X<bareword>
+
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
"barewords". As with filehandles and labels, a bareword that consists
entirely of lowercase letters risks conflict with future reserved
-words, and if you use the B<-w> switch, Perl will warn you about any
-such words. Some people may wish to outlaw barewords entirely. If you
+words, and if you use the C<use warnings> pragma or the B<-w> switch,
+Perl will warn you about any such words. Perl limits barewords (like
+identifiers) to about 250 characters. Future versions of Perl are likely
+to eliminate these arbitrary limitations.
+
+Some people may wish to outlaw barewords entirely. If you
say
use strict 'subs';
end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
-Array variables are interpolated into double-quoted strings by joining all
-the elements of the array with the delimiter specified in the C<$">
-variable (C<$LIST_SEPARATOR> in English), space by default. The following
-are equivalent:
+=head3 Array Interpolation
+X<array, interpolation> X<interpolation, array> X<$">
+
+Arrays and slices are interpolated into double-quoted strings
+by joining the elements with the delimiter specified in the C<$">
+variable (C<$LIST_SEPARATOR> if "use English;" is specified),
+space by default. The following are equivalent:
- $temp = join($",@ARGV);
+ $temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution)
-there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as
+there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
@foo)? If @foo doesn't otherwise exist, then it's obviously a
character class. If @foo exists, Perl takes a good guess about C<[bar]>,
and is almost always right. If it does guess wrong, or if you're just
plain paranoid, you can force the correct interpretation with curly
-brackets as above.
-
-A line-oriented form of quoting is based on the shell "here-doc"
-syntax. Following a C<E<lt>E<lt>> you specify a string to terminate
-the quoted material, and all lines following the current line down to
-the terminating string are the value of the item. The terminating
-string may be either an identifier (a word), or some quoted text. If
-quoted, the type of quotes you use determines the treatment of the
-text, just as in regular quoting. An unquoted identifier works like
-double quotes. There must be no space between the C<E<lt>E<lt>> and
-the identifier. (If you put a space it will be treated as a null
-identifier, which is valid, and matches the first empty line.) The
-terminating string must appear by itself (unquoted and with no
-surrounding whitespace) on the terminating line.
-
- print <<EOF;
- The price is $Price.
- EOF
-
- print <<"EOF"; # same as above
- The price is $Price.
- EOF
-
- print <<`EOC`; # execute commands
- echo hi there
- echo lo there
- EOC
-
- print <<"foo", <<"bar"; # you can stack them
- I said foo.
- foo
- I said bar.
- bar
-
- myfunc(<<"THIS", 23, <<'THAT');
- Here's a line
- or two.
- THIS
- and here's another.
- THAT
-
-Just don't forget that you have to put a semicolon on the end
-to finish the statement, as Perl doesn't know you're not going to
-try to do this:
-
- print <<ABC
- 179231
- ABC
- + 20;
+braces as above.
+If you're looking for the information on how to use here-documents,
+which used to be here, that's been moved to
+L<perlop/Quote and Quote-like Operators>.
=head2 List value constructors
+X<list>
List values are denoted by separating individual values by commas
(and enclosing the list in parentheses where precedence requires it):
(LIST)
-In a context not requiring a list value, the value of the list
-literal is the value of the final element, as with the C comma operator.
-For example,
+In a context not requiring a list value, the value of what appears
+to be a list literal is simply the value of the final element, as
+with the C comma operator. For example,
@foo = ('cc', '-E', $bar);
-assigns the entire list value to array foo, but
+assigns the entire list value to array @foo, but
$foo = ('cc', '-E', $bar);
-assigns the value of variable bar to variable foo. Note that the value
-of an actual array in a scalar context is the length of the array; the
-following assigns the value 3 to $foo:
+assigns the value of variable $bar to the scalar variable $foo.
+Note that the value of an actual array in scalar context is the
+length of the array; the following assigns the value 3 to $foo:
@foo = ('cc', '-E', $bar);
- $foo = @foo; # $foo gets 3
+ $foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:
@foo = (
- 1,
- 2,
- 3,
+ 1,
+ 2,
+ 3,
);
+To use a here-document to assign an array, one line per element,
+you might use an approach like this:
+
+ @sauces = <<End_Lines =~ m/(\S.*\S)/g;
+ normal tomato
+ spicy tomato
+ green chile
+ pesto
+ white wine
+ End_Lines
+
LISTs do automatic interpolation of sublists. That is, when a LIST is
-evaluated, each element of the list is evaluated in a list context, and
+evaluated, each element of the list is evaluated in list context, and
the resulting list value is interpolated into LIST just as if each
individual element were a member of LIST. Thus arrays and hashes lose their
identity in a LIST--the list
contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub
-called in a list context, followed by the key/value pairs of %glarch.
+called in list context, followed by the key/value pairs of %glarch.
To make a list reference that does I<NOT> interpolate, see L<perlref>.
The null list is represented by (). Interpolating it in a list
interpolating an array with no elements is the same as if no
array had been interpolated at that point.
+This interpolation combines with the facts that the opening
+and closing parentheses are optional (except when necessary for
+precedence) and lists may end with an optional comma to mean that
+multiple commas within lists are legal syntax. The list C<1,,3> is a
+concatenation of two lists, C<1,> and C<3>, the first of which ends
+with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
+similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
+we'd advise you to use this obfuscation.
+
A list value may also be subscripted like a normal array. You must
put the list in parentheses to avoid ambiguity. For example:
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
-You may assign to C<undef> in a list. This is useful for throwing
-away some of the return values of a function:
-
- ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
-
-Lists may be assigned to if and only if each element of the list
-is legal to assign to:
+Lists may be assigned to only when each element of the list
+is itself legal to assign to:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
-List assignment in a scalar context returns the number of elements
+An exception to this is that you may assign to C<undef> in a list.
+This is useful for throwing away some of the return values of a
+function:
+
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+
+List assignment in scalar context returns the number of elements
produced by the expression on the right side of the assignment:
- $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
- $x = (($foo,$bar) = f()); # set $x to f()'s return count
+ $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
+ $x = (($foo,$bar) = f()); # set $x to f()'s return count
-This is very handy when you want to do a list assignment in a Boolean
+This is handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.
-The final element may be an array or a hash:
+It's also the source of a useful idiom for executing a function or
+performing an operation in list context and then counting the number of
+return values, by assigning to an empty list and then using that
+assignment in scalar context. For example, this code:
+
+ $count = () = $string =~ /\d+/g;
+
+will place into $count the number of digit groups found in $string.
+This happens because the pattern match is in list context (since it
+is being assigned to the empty list), and will therefore return a list
+of all matching parts of the string. The list assignment in scalar
+context will translate that into the number of elements (here, the
+number of times the pattern matched) and assign that to $count. Note
+that simply using
+
+ $count = $string =~ /\d+/g;
+
+would not have worked, since a pattern match in scalar context will
+only return true or false, rather than a count of matches.
+
+The final element of a list assignment may be an array or a hash:
($a, $b, @rest) = split;
my($a, $b, %rest) = @_;
You can actually put an array or hash anywhere in the list, but the first one
-in the list will soak up all the values, and anything after it will get
-a null value. This may be useful in a local() or my().
+in the list will soak up all the values, and anything after it will become
+undefined. This may be useful in a my() or local().
-A hash literal contains pairs of values to be interpreted
-as a key and a value:
+A hash can be initialized using a literal list holding pairs of
+items to be interpreted as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
-While literal lists and named arrays are usually interchangeable, that's
+While literal lists and named arrays are often interchangeable, that's
not the case for hashes. Just because you can subscript a list value like
a normal array does not mean that you can subscript a list value as a
hash. Likewise, hashes included as parts of other lists (including
parameters lists and return lists from functions) always flatten out into
key/value pairs. That's why it's good to use references sometimes.
-It is often more readable to use the C<=E<gt>> operator between key/value
-pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
+It is often more readable to use the C<< => >> operator between key/value
+pairs. The C<< => >> operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to be
-interpreted as a string--if it's a bareword that would be a legal identifier.
-This makes it nice for initializing hashes:
+interpreted as a string if it's a bareword that would be a legal simple
+identifier. C<< => >> doesn't quote compound identifiers, that contain
+double colons. This makes it nice for initializing hashes:
%map = (
- red => 0x00f,
- blue => 0x0f0,
- green => 0xf00,
+ red => 0x00f,
+ blue => 0x0f0,
+ green => 0xf00,
);
or for initializing hash references to be used as records:
$rec = {
- witch => 'Mable the Merciless',
- cat => 'Fluffy the Ferocious',
- date => '10/31/1776',
+ witch => 'Mable the Merciless',
+ cat => 'Fluffy the Ferocious',
+ date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
$field = $query->radio_group(
- name => 'group_name',
+ name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
+If a key appears more than once in the initializer list of a hash, the last
+occurrence wins:
+
+ %circle = (
+ center => [5, 10],
+ center => [27, 9],
+ radius => 100,
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+ # same as
+ %circle = (
+ center => [27, 9],
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+This can be used to provide overridable configuration defaults:
+
+ # values in %args take priority over %config_defaults
+ %config = (%config_defaults, %args);
+
+=head2 Subscripts
+
+An array can be accessed one scalar at a
+time by specifying a dollar sign (C<$>), then the
+name of the array (without the leading C<@>), then the subscript inside
+square brackets. For example:
+
+ @myarray = (5, 50, 500, 5000);
+ print "The Third Element is", $myarray[2], "\n";
+
+The array indices start with 0. A negative subscript retrieves its
+value from the end. In our example, C<$myarray[-1]> would have been
+5000, and C<$myarray[-2]> would have been 500.
+
+Hash subscripts are similar, only instead of square brackets curly brackets
+are used. For example:
+
+ %scientists =
+ (
+ "Newton" => "Isaac",
+ "Einstein" => "Albert",
+ "Darwin" => "Charles",
+ "Feynman" => "Richard",
+ );
+
+ print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
+
+You can also subscript a list to get a single element from it:
+
+ $dir = (getpwnam("daemon"))[7];
+
+=head2 Multi-dimensional array emulation
+
+Multidimensional arrays may be emulated by subscripting a hash with a
+list. The elements of the list are joined with the subscript separator
+(see L<perlvar/$;>).
+
+ $foo{$a,$b,$c}
+
+is equivalent to
+
+ $foo{join($;, $a, $b, $c)}
+
+The default subscript separator is "\034", the same as SUBSEP in B<awk>.
+
+=head2 Slices
+X<slice> X<array, slice> X<hash, slice>
+
+A slice accesses several elements of a list, an array, or a hash
+simultaneously using a list of subscripts. It's more convenient
+than writing out the individual elements as a list of separate
+scalar values.
+
+ ($him, $her) = @folks[0,-1]; # array slice
+ @them = @folks[0 .. 3]; # array slice
+ ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
+ ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
+
+Since you can assign to a list of variables, you can also assign to
+an array or hash slice.
+
+ @days[3..5] = qw/Wed Thu Fri/;
+ @colors{'red','blue','green'}
+ = (0xff0000, 0x0000ff, 0x00ff00);
+ @folks[0, -1] = @folks[-1, 0];
+
+The previous assignments are exactly equivalent to
+
+ ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
+ ($colors{'red'}, $colors{'blue'}, $colors{'green'})
+ = (0xff0000, 0x0000ff, 0x00ff00);
+ ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
+
+Since changing a slice changes the original array or hash that it's
+slicing, a C<foreach> construct will alter some--or even all--of the
+values of the array or hash.
+
+ foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
+
+ foreach (@hash{qw[key1 key2]}) {
+ s/^\s+//; # trim leading whitespace
+ s/\s+$//; # trim trailing whitespace
+ s/(\w+)/\u\L$1/g; # "titlecase" words
+ }
+
+A slice of an empty list is still an empty list. Thus:
+
+ @a = ()[1,0]; # @a has no elements
+ @b = (@a)[0,1]; # @b has no elements
+
+But:
+
+ @a = (1)[1,0]; # @a has two elements
+ @b = (1,undef)[1,0,2]; # @b has three elements
+
+More generally, a slice yields the empty list if it indexes only
+beyond the end of a list:
+
+ @a = (1)[ 1,2]; # @a has no elements
+ @b = (1)[0,1,2]; # @b has three elements
+
+This makes it easy to write loops that terminate when a null list
+is returned:
+
+ while ( ($home, $user) = (getpwent)[7,0]) {
+ printf "%-8s %s\n", $user, $home;
+ }
+
+As noted earlier in this document, the scalar sense of list assignment
+is the number of elements on the right-hand side of the assignment.
+The null list contains no elements, so when the password file is
+exhausted, the result is 0, not 2.
+
+Slices in scalar context return the last item of the slice.
+
+ @a = qw/first second third/;
+ %h = (first => 'A', second => 'B');
+ $t = @a[0, 1]; # $t is now 'second'
+ $u = @h{'first', 'second'}; # $u is now 'B'
+
+If you're confused about why you use an '@' there on a hash slice
+instead of a '%', think of it like this. The type of bracket (square
+or curly) governs whether it's an array or a hash being looked at.
+On the other hand, the leading symbol ('$' or '@') on the array or
+hash indicates whether you are getting back a singular value (a
+scalar) or a plural one (a list).
+
+=head3 Key/Value Hash Slices
+
+Starting in Perl 5.20, a hash slice operation
+with the % symbol is a variant of slice operation
+returning a list of key/value pairs rather than just values:
+
+ %h = (blonk => 2, foo => 3, squink => 5, bar => 8);
+ %subset = %h{'foo', 'bar'}; # key/value hash slice
+ # %subset is now (foo => 3, bar => 8)
+
+However, the result of such a slice cannot be localized, deleted or used
+in assignment. These are otherwise very much consistent with hash slices
+using the @ symbol.
+
+=head3 Index/Value Array Slices
+
+Similar to key/value hash slices (and also introduced
+in Perl 5.20), the % array slice syntax returns a list
+of index/value pairs:
+
+ @a = "a".."z";
+ @list = %a[3,4,6];
+ # @list is now (3, "d", 4, "e", 6, "g")
+
=head2 Typeglobs and Filehandles
+X<typeglob> X<filehandle> X<*>
Perl uses an internal type called a I<typeglob> to hold an entire
symbol table entry. The type prefix of a typeglob is a C<*>, because
of this. Strange though this may seem, this is the basis for the whole
module import/export system.
-Another use for typeglobs is to to pass filehandles into a function or
+Another use for typeglobs is to pass filehandles into a function or
to create new filehandles. If you need to use a typeglob to save away
a filehandle, do it this way:
For example:
sub newopen {
- my $path = shift;
- local *FH; # not my!
- open (FH, $path) or return undef;
- return *FH;
+ my $path = shift;
+ local *FH; # not my!
+ open (FH, $path) or return undef;
+ return *FH;
}
$fh = newopen('/etc/passwd');
-Now that we have the *foo{THING} notation, typeglobs aren't used as much
+Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
for filehandle manipulations, although they're still needed to pass brand
-new file and directory handles into or out of functions. That's because
-*HANDLE{IO} only works if HANDLE has already been used as a handle.
-In other words, *FH can be used to create new symbol table entries,
-but *foo{THING} cannot.
-
-Another way to create anonymous filehandles is with the IO::Handle
-module and its ilk. These modules have the advantage of not hiding
-different types of the same name during the local(). See the bottom of
-L<perlfunc/open()> for an example.
-
-See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more
-discussion on typeglobs and the *foo{THING} syntax.
+new file and directory handles into or out of functions. That's because
+C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
+In other words, C<*FH> must be used to create new symbol table entries;
+C<*foo{THING}> cannot. When in doubt, use C<*FH>.
+
+All functions that are capable of creating filehandles (open(),
+opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
+automatically create an anonymous filehandle if the handle passed to
+them is an uninitialized scalar variable. This allows the constructs
+such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
+create filehandles that will conveniently be closed automatically when
+the scope ends, provided there are no other references to them. This
+largely eliminates the need for typeglobs when opening filehandles
+that must be passed around, as in the following example:
+
+ sub myopen {
+ open my $fh, "@_"
+ or die "Can't open '@_': $!";
+ return $fh;
+ }
+
+ {
+ my $f = myopen("</etc/motd");
+ print <$f>;
+ # $f implicitly closed here
+ }
+
+Note that if an initialized scalar variable is used instead the
+result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
+to C<open( *{'zzz'}, ...)>.
+C<use strict 'refs'> forbids such practice.
+
+Another way to create anonymous filehandles is with the Symbol
+module or with the IO::Handle module and its ilk. These modules
+have the advantage of not hiding different types of the same name
+during the local(). See the bottom of L<perlfunc/open> for an
+example.
+
+=head1 SEE ALSO
+
+See L<perlvar> for a description of Perl's built-in variables and
+a discussion of legal variable names. See L<perlref>, L<perlsub>,
+and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
+the C<*foo{THING}> syntax.
https://perl5.git.perl.org / perl5.git / blobdiff