Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

The Perl parser will expand $variable and @variable references in regular expressions unless the delimiter is a single quote. Remember, too, that the right-hand side of a s/// substitution is considered a double-quoted string (see the perlop manpage for more details). Remember also that any regexp special characters will be acted on unless you precede the substitution with Q. Here's an example:

    $string = "to die?";
    $lhs = "die?";
    $rhs = "sleep no more";

    $string =~ s/Q$lhs/$rhs/;
    # $string is now "to sleep no more"

Without the Q, the regexp would also spuriously match ``di''.