perldata.pod

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Numeric literals are specified in any of the following floating point or
integer formats:

    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

You are allowed to use underscores (underbars) in numeric literals
between digits for legibility.  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 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.

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 $Z<>100."
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, 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, 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>

B<Note:> Version Strings (v-strings) have been deprecated.  They will
be removed in some future release after Perl 5.8.1.  The marginal
benefits of v-strings were greatly outweighed by the potential for
Surprise and Confusion.

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
will not be interpolated into strings.  If there is no current package
(due to an empty C<package;> directive), __PACKAGE__ is the undefined
value.
X<__FILE__> X<__LINE__> X<__PACKAGE__> 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__ but 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
contents after __DATA__.  It is the program's responsibility to
C<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 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 C<use warnings> pragma or the B<-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 C<no strict 'subs'>.

=head3 Array Joining Delimiter
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);
    system "echo $temp";

    system "echo @ARGV";

Within search patterns (which also undergo double-quotish substitution)
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
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 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 = (
        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 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

    (@foo,@bar,&SomeSub,%glarch)

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 I<NOT> interpolate, see L<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.

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:

    # 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 C<undef> in a list.
This is useful for throwing away some of the return values of a
function: