Could it be that the one-time compilation of the regex somehow causes the closure?

That's exactly what it is. Your regex() sub isn't the closure, the code in the regular expression code assertion is.

Also, saying local $ret = ''; breaks under strict:

Well of course it does. :-) (I should have mentioned that though.) Use our $ret = ''; or declare $ret with use vars qw( $ret ); instead. And keep in mind that whichever way you choose to declare it, it's still a global variable.

-sauoq
"My two cents aren't worth a dime.";