perldata.pod

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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.

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."

    $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 (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:

    $who = "Larry";
    print PASSWD "${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.

A literal of the form C<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 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 UTF-8 encoded 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.  The C<$^V> special variable also contains the
running Perl interpreter's version in this form.  See L<perlvar/$^V>.

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.

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 toplevel 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.

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'>.

Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the C<$">
variable (C<$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 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.

A line-oriented form of quoting is based on the shell "here-document"
syntax.  Following a C<< << >> 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<< << >> and
the identifier, unless the identifier is quoted.  (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;

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.
    FINIS

If you use a here-doc within a delimited construct, such as in C<s///eg>,
the quoted material must come on the lines following the final delimiter.
So instead of

    s/this/<<E . 'that'
    the other
    E
     . 'more '/eg;

you have to write

    s/this/<<E . 'that' 
     . 'more '/eg; 
    the other 
    E 

If the terminating identifier is on the last line of the program, you
must be sure there is a newline after it; otherwise, Perl will give the
warning B<Can't find string terminator "END" anywhere before EOF...>.

Additionally, the quoting rules for the identifier are not related to
Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
in place of C<''> and C<"">, and the only interpolation is for backslashing
the quoting character:

    print << "abc\"def";
    testing...
    abc"def

Finally, quoted strings cannot span multiple lines.  The general rule is
that the identifier must be a string literal.  Stick with that, and you
should be safe.

=head2 List value constructors

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>.