in reply to creating qr from existing regex

Not an answer, just a comment. Do I get it right:

It doesn't make much sense to me, but I haven't worked with MongoDB, hence not an answer, but a promised comment. The reason you get an undef in OP, is because $re are 2 separate lexical variables, 1 per each own small block. They are declared with my and destroyed at closing curly bracket. What's assigned to $_[0], in next line, is presumably global, completely different $re, and undefined at that time. You don't use strict;, do you? I think this change

my $re; if($2 eq 'i'){ $re = qr/$_[0]/i; }else{ $re = qr/$_[0]/; } $_[0] =$re;

would give you the result you were hoping for.

Edit: Fixed numeric comparison operator to string comparison eq (thanks, poj).