Learning Perl, Fourth Edition

Interpolating Arrays into Strings

Like scalars, array values may be interpolated into a double-quoted string. Elements of an array are automatically separated by spaces^[74] upon interpolation:

    @rocks = qw{ flintstone slate rubble };
    print "quartz @rocks limestone\n";  # prints five rocks separated by spaces

There are no extra spaces added before or after an interpolated array; if you want those, you’ll have to put them in yourself:

    print "Three rocks are: @rocks.\n";
    print "There's nothing in the parens (@empty) here.\n";

If you forget that arrays interpolate like this, you’ll be surprised when you put an email address into a double-quoted string. For historical reasons,^[75] this is a fatal error at compile time:

    $email = "[e-mail]";  # WRONG! Tries to interpolate @bedrock
    $email = "fred\@bedrock.edu"; # Correct
    $email = '[e-mail]';  # Another way to do that

However, in versions of Perl 5 soon to be released as we write this, the behavior of an unseen array variable will become similar to an unseen scalar variable, i.e., replaced with an empty string with a warning if warnings are enabled. The Perl developers apparently figure that 10 years of fatality are enough warning.

A single element of an array will be replaced by its value as you’d expect:

    @fred = qw(hello dolly);
    $y = 2;
    $x = "This is $fred[1]'s place";    # "This is dolly's place"
    $x = "This is $fred[$y-1]'s place"; # same thing

The index expression is evaluated as an ordinary expression, as if it were outside a string. ...