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 perlop for more details). Remember also that any regex 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 regex would also spuriously match "di".