Note that if $key is a user-provided value, doing this is not a good idea. At the very least you would want to enclose the whole thing in an eval in case $key contains some characters that upset the match operator (for example, if $key contains **, the program will die with "nested *?+ in regexp"). In the worst case, $key could contain something that makes Perl execute arbitrary code. So for doing something like this, you have to make sure that $key is properly untainted.

Update: You could also use \Q, as in "string" =~ m/ \Q$key /

--ZZamboni