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Scalar value constructors

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Scalar value constructors

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

    0xffff              # hex
    0377                # octal
    4_294_967_296       # underline for legibility

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 ``\''' and ``\\''). The usual Unix backslash rules apply for making characters such as newline, tab, etc., as well as some more exotic forms. See 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 hex and 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 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 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,


can be written as


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 Packages).

Three special literals are __FILE__, __LINE__, and __PACKAGE__, which 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 a 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 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 -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 'subs'.

Array variables are interpolated into double-quoted strings by joining all the elements of the array 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 a bad 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 brackets as above.

A line-oriented form of quoting is based on the shell ``here-doc'' 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;

Source: Perl data types
Copyright: Larry Wall, et al.
Next: List value constructors

Previous: Scalar values

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