Go with lc.

If this hash only holds filehandles and the rest of the user info is stored elsewhere then just store lowercased nicks and look up lowercased nicks, like Juerd++ suggested.

If you really have reason not to do that, then you should use an array (or a pair of arrays) since that's more efficient to step through than a hash. However, remember that avoiding the expensive stepping through arrays to find a single value is what hashes were made for. Anyway, if you stick with that approach, do my $nick = lc shift; first so you don't need to repeatedly lowercase the nick within the loop, then use return $nick_check if $nick eq lc $nick_check; rather than a regex.

Makeshifts last the longest.