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perldata ()
  • >> perldata (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perldata (1) ( Разные man: Команды и прикладные программы пользовательского уровня )


         perldata - Perl data types


         Variable names
         Perl has three built-in data types: scalars, arrays of
         scalars, and associative arrays of scalars, known as
         "hashes".  Normal arrays are ordered lists of scalars
         indexed by number, starting with 0 and with negative
         subscripts counting from the end.  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.  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.  Usually
         this name is a single 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 `::' (or by the
         slightly archaic `''); all but the last are interpreted as
         names of packages, to locate the namespace in which to look
         up the final identifier (see the Packages entry in the
         perlmod manpage 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 the perlref manpage.
         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 `$' (see the perlop manpage and the perlre
         manpage).  In addition, several special variables that
         provide windows into the inner working of Perl have names
         containing punctuation characters and control characters.
         These are documented in the perlvar manpage.
         Scalar values are always named with '$', 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.
             $days               # the simple scalar value "days"
             $days[28]           # the 29th element of array @days
             $days{'Feb'}        # the 'Feb' value from hash %days
             $#days              # the last index of array @days
         Entire arrays (and slices of arrays and hashes) are denoted
         by '@', which works much like the word "these" or "those"
         does in English, in that it indicates multiple values are
             @days               # ($days[0], $days[1],... $days[n])
             @days[3,4,5]        # same as ($days[3],$days[4],$days[5])
             @days{'a','c'}      # same as ($days{'a'},$days{'c'})
         Entire hashes are denoted by '%':
             %days               # (key1, val1, key2, val2 ...)
         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 $foo and @foo are two different
         variables.  It also means that `$foo[1]' is a part of @foo,
         not a part of $foo.  This may seem a bit weird, but that's
         okay, because it is weird.
         Because variable references always start with '$', '@', 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
         `open(LOG,'logfile')' rather than `open(log,'logfile')'.
         Using uppercase filehandles also improves readability and
         protects you from conflict with future reserved words.  Case
         is significant--"FOO", "Foo", and "foo" are all different
         names.  Names that start with a letter or underscore may
         also contain digits and underscores.
         It is possible to replace such an alphanumeric name with an
         expression that returns a reference to the appropriate type.
         For a description of this, see the perlref manpage.
         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.,  `$%' or `$$'.  (Most of
         these one character names have a predefined significance to
         Perl.  For instance, `$$' is the current process id.)
         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: 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 certain operations based on whether the expected
         return value is 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 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 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 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 righthand side in list context.
         When you use the `use warnings' pragma or Perl's -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 `"fred";' or `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 the wantarray entry in the perlfunc manpage for
         how you would dynamically discern your function's calling
         Scalar values
         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 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 where no conversion to a string or a
         number is ever performed.
         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 `""'.  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 the perlref manpage.  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.
         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 -w
         noises).  That's because strings that aren't numbers count
         as 0, just as they do in awk:
             if ($str == 0 && $str ne "0")  {
                 warn "That doesn't look like a number";
         That method may be best because otherwise you won't treat
         IEEE notations like `NaN' or `Infinity' properly.  At other
         times, you might prefer to determine whether string data can
         be used numerically by calling the POSIX:\fIs0:strtod()
         function or by inspecting your string with a regular
         expression (as documented in the perlre manpage).
             warn "has nondigits"        if     /\D/;
             warn "not a natural number" unless /^\d+$/;             # rejects -3
             warn "not an integer"       unless /^-?\d+$/;           # rejects +3
             warn "not an integer"       unless /^[+-]?\d+$/;
             warn "not a decimal number" unless /^-?\d+\.?\d*$/;     # rejects .2
             warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
             warn "not a C float"
                 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
         The length of an array is a scalar value.  You may find the
         length of array @days by evaluating `$#days', as in csh.
         Technically speaking, this isn't the length of the array;
         it's the subscript of the last element, since there is
         ordinarily a 0th element.  Assigning to `$#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.  (It used to do so in Perl 4, but we had
         to break this to make sure destructors were called when
         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:
             @whatever = ();
             $#whatever = -1;
         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:
             scalar(@whatever) == $#whatever - $[ + 1;
         Version 5 of Perl changed the semantics of `$[': files that
         don't set the value of `$[' no longer need to worry about
         whether another file changed its value.  (In other words,
         use of `$[' is deprecated.)  So in general you can assume
             scalar(@whatever) == $#whatever + 1;
         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 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 `"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
         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:
             keys(%users) = 1000;                # allocate 1024 buckets
         Scalar value constructors
         Numeric literals are specified in any of the following
         floating point or integer formats:
             .23E-10             # a very small number
             4_294_967_296       # underline for legibility
             0xff                # hex
             0377                # octal
             0b011011            # binary
         String literals are usually delimited by either single or
         double 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 `\'' and `\\').  The usual C-style
         backslash rules apply for making characters such as newline,
         tab, etc., as well as some more exotic forms.  See the Quote
         and Quote-like Operators entry in the perlop manpage for a
         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 the hex entry in the
         perlfunc manpage and the oct entry in the perlfunc manpage
         for more details.
         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 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 $100."
             $Price = '$100';    # not interpreted
             print "The price is $Price.\n";     # interpreted
         As in some shells, you can enclose the variable name in
         braces to disambiguate it from following alphanumerics.  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:
             $who = "Larry";
             print PASSWD "${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 $whospeak,
         a `$who::0', and a `$who's' variable.  The last two would be
         the $0 and the $s variables in the (presumably) non-existent
         package `who'.
         In fact, an identifier within such curlies is forced to be a
         string, as is any simple identifier within a hash subscript.
         Neither need quoting.  Our earlier example, `$days{'Feb'}'
         can be written as `$days{Feb}' and the quotes will be
         assumed automatically.  But anything more complicated in the
         subscript will be interpreted as an expression.
         A literal of the form `v1.20.300.4000' is parsed as a string
         composed of characters with the specified ordinals.  This
         provides an alternative, more readable way to construct
         strings, rather than use the somewhat less readable
         interpolation form `"\x{1}\x{14}\x{12c}\x{fa0}"'.  This is
         useful for representing Unicode strings, and for comparing
         version "numbers" using the string comparison operators,
         `cmp', `gt', `lt' etc.  If there are two or more dots in the
         literal, the leading `v' may be omitted.
             print v9786;              # prints UTF-8 encoded SMILEY, "\x{263a}"
             print v102.111.111;       # prints "foo"
             print 102.111.111;        # same
         Such literals are accepted by both `require' and `use' for
         doing a version check.  The `$^V' special variable also
         contains the running Perl interpreter's version in this
         form.  See the section on "$^V" in the perlvar manpage.
         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 will not be interpolated into
         strings.  If there is no current package (due to an empty
         `package;' directive), __PACKAGE__ is the undefined value.
         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
         Text after __DATA__ but may be read via the filehandle
         `PACKNAME::DATA', where `PACKNAME' is the package that was
         current when the __DATA__ token was encountered.  The
         filehandle is left open pointing to the contents after
         __DATA__.  It is the program's responsibility to `close
         DATA' when it is done reading from it.  For compatibility
         with older scripts written before __DATA__ was introduced,
         __END__ behaves like __DATA__ in the toplevel script (but
         not in files loaded with `require' or `do') and leaves the
         remaining contents of the file accessible via `main::DATA'.
         See the SelfLoader manpage 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.
         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 `use
         warnings' pragma or the -w switch, Perl will warn you about
         any such words.  Some people may wish to outlaw barewords
         entirely.  If you say
             use strict 'subs';
         then any bareword that would NOT be interpreted as a
         subroutine call produces a compile-time error instead.  The
         restriction lasts to the end of the enclosing block.  An
         inner block may countermand this by saying `no strict
         Arrays and slices are interpolated into double-quoted
         strings by joining the elements with the delimiter specified
         in the `$"' variable (`$LIST_SEPARATOR' in English), space
         by default.  The following are equivalent:
             $temp = join($", @ARGV);
             system "echo $temp";
             system "echo @ARGV";
         Within search patterns (which also undergo double-quotish
         substitution) there is an unfortunate ambiguity:  Is
         `/$foo[bar]/' to be interpreted as `/${foo}[bar]/' (where
         `[bar]' is a character class for the regular expression) or
         as `/${foo[bar]}/' (where `[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 `[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 braces as above.
         A line-oriented form of quoting is based on the shell
         "here-document" syntax.  Following a `<<' 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 `<<' 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.
                 print <<"EOF";  # same as above
             The price is $Price.
                 print <<`EOC`;  # execute commands
             echo hi there
             echo lo there
                 print <<"foo", <<"bar"; # you can stack them
             I said foo.
             I said bar.
                 myfunc(<<"THIS", 23, <<'THAT');
             Here's a line
             or two.
             and here's another.
         Just don't forget that you have to put a semicolon on the
         end to finish the statement, as Perl doesn't know you're not
         going to try to do this:
                 print <<ABC
                 + 20;
         If you want your here-docs to be indented with the rest of
         the code, you'll need to remove leading whitespace from each
         line manually:
             ($quote = <<'FINIS') =~ s/^\s+//gm;
                 The Road goes ever on and on,
                 down from the door where it began.
         List value constructors
         List values are denoted by separating individual values by
         commas (and enclosing the list in parentheses where
         precedence requires it):
         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
             $foo = ('cc', '-E', $bar);
         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
         You may have an optional comma before the closing
         parenthesis of a list literal, so that you can say:
             @foo = (
         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
                 white wine
         LISTs do automatic interpolation of sublists.  That is, when
         a LIST is 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 list context,
         followed by the key/value pairs of %glarch.  To make a list
         reference that does NOT interpolate, see the perlref
         The null list is represented by ().  Interpolating it in a
         list has no effect.  Thus ((),(),()) is equivalent to ().
         Similarly, interpolating an array with no elements is the
         same as if no array had been interpolated at that point.
         A list value may also be subscripted like a normal array.
         You must put the list in parentheses to avoid ambiguity.
         For example:
             # Stat returns list value.
             $time = (stat($file))[8];
             # SYNTAX ERROR HERE.
             $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES
             # Find a hex digit.
             $hexdigit = ('a','b','c','d','e','f')[$digit-10];
             # A "reverse comma operator".
             return (pop(@foo),pop(@foo))[0];
         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'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
         An exception to this is that you may assign to `undef' in a
         list.  This is useful for throwing away some of the return
         values of a function:
             ($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
             $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
             $x = (($foo,$bar) = f());           # set $x to f()'s return count
         This is handy when you want to do a list assignment in a
         Boolean context, because most list functions return a null
         list when finished, which when assigned produces a 0, which
         is interpreted as FALSE.
         The final element may be an array or a hash:
             ($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 become undefined.  This may be
         useful in a my() or local().
         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 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 `=>' operator between
         key/value pairs.  The `=>' 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:
             %map = (
                          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',
         or for using call-by-named-parameter to complicated
            $field = $query->radio_group(
                        name      => 'group_name',
                        values    => ['eenie','meenie','minie'],
                        default   => 'meenie',
                        linebreak => 'true',
                        labels    => \%labels
         Note that just because a hash is initialized in that order
         doesn't mean that it comes out in that order.  See the sort
         entry in the perlfunc manpage for examples of how to arrange
         for an output ordering.
         A common way to access an array or a hash is one scalar
         element at a time.  You can also subscript a list to get a
         single element from it.
             $whoami = $ENV{"USER"};             # one element from the hash
             $parent = $ISA[0];                  # one element from the array
             $dir    = (getpwnam("daemon"))[7];  # likewise, but with list
         A slice accesses several elements of a list, an array, or a
         hash simultaneously using a list of subscripts.  It's more
         convenient than writing out the individual elements as a
         list of separate scalar values.
             ($him, $her)   = @folks[0,-1];              # array slice
             @them          = @folks[0 .. 3];            # array slice
             ($who, $home)  = @ENV{"USER", "HOME"};      # hash slice
             ($uid, $dir)   = (getpwnam("daemon"))[2,7]; # list slice
         Since you can assign to a list of variables, you can also
         assign to an array or hash slice.
             @days[3..5]    = qw/Wed Thu Fri/;
                            = (0xff0000, 0x0000ff, 0x00ff00);
             @folks[0, -1]  = @folks[-1, 0];
         The previous assignments are exactly equivalent to
             ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
             ($colors{'red'}, $colors{'blue'}, $colors{'green'})
                            = (0xff0000, 0x0000ff, 0x00ff00);
             ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
         Since changing a slice changes the original array or hash
         that it's slicing, a `foreach' construct will alter some--or
         even all--of the values of the array or hash.
             foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
             foreach (@hash{keys %hash}) {
                 s/^\s+//;           # trim leading whitespace
                 s/\s+$//;           # trim trailing whitespace
                 s/(\w+)/\u\L$1/g;   # "titlecase" words
         A slice of an empty list is still an empty list.  Thus:
             @a = ()[1,0];           # @a has no elements
             @b = (@a)[0,1];         # @b has no elements
             @c = (0,1)[2,3];        # @c has no elements
             @a = (1)[1,0];          # @a has two elements
             @b = (1,undef)[1,0,2];  # @b has three elements
         This makes it easy to write loops that terminate when a null
         list is returned:
             while ( ($home, $user) = (getpwent)[7,0]) {
                 printf "%-8s %s\n", $user, $home;
         As noted earlier in this document, the scalar sense of list
         assignment is the number of elements on the right-hand side
         of the assignment.  The null list contains no elements, so
         when the password file is exhausted, the result is 0, not 2.
         If you're confused about why you use an '@' there on a hash
         slice instead of a '%', think of it like this.  The type of
         bracket (square or curly) governs whether it's an array or a
         hash being looked at.  On the other hand, the leading symbol
         ('$' or '@') on the array or hash indicates whether you are
         getting back a singular value (a scalar) or a plural one (a
         Typeglobs and Filehandles
         Perl uses an internal type called a typeglob to hold an
         entire symbol table entry.  The type prefix of a typeglob is
         a `*', because it represents all types.  This used to be the
         preferred way to pass arrays and hashes by reference into a
         function, but now that we have real references, this is
         seldom needed.
         The main use of typeglobs in modern Perl is create symbol
         table aliases.  This assignment:
             *this = *that;
         makes $this an alias for $that, @this an alias for @that,
         %this an alias for %that, &this an alias for &that, etc.
         Much safer is to use a reference.  This:
             local *Here::blue = \$There::green;
         temporarily makes $Here::blue an alias for $There::green,
         but doesn't make @Here::blue an alias for @There::green, or
         %Here::blue an alias for %There::green, etc.  See the Symbol
         Tables entry in the perlmod manpage for more examples of
         this.  Strange though this may seem, this is the basis for
         the whole module import/export system.
         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:
             $fh = *STDOUT;
         or perhaps as a real reference, like this:
             $fh = \*STDOUT;
         See the perlsub manpage for examples of using these as
         indirect filehandles in functions.
         Typeglobs are also a way to create a local filehandle using
         the local() operator.  These last until their block is
         exited, but may be passed back.  For example:
             sub newopen {
                 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 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' must be used to create new
         symbol table entries; `*foo{THING}' cannot.  When in doubt,
         use `*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 `open(my
         $fh, ...)' and `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
         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 the open() entry in the perlfunc manpage for an example.


         See the perlvar manpage for a description of Perl's built-in
         variables and a discussion of legal variable names.  See the
         perlref manpage, the perlsub manpage, and the Symbol Tables
         entry in the perlmod manpage for more discussion on
         typeglobs and the `*foo{THING}' syntax.

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