Google / Super Search is your friend, just a selection:

• Case insensitivity in a hash... Futile effort?
• Hash Keys case independent
• Not case sensitive hash?
• On creating hash whose keys aren't case-sensitive
• case-insensitive hash keys

The only thing I have to add at the moment is that as of Perl 5.16 in addition to lc there is fc which might be useful if you have Unicode data. (Update: Didn't see Your Mother's post before posting.)