|We don't bite newbies here... much|
What is the purpose? If it's for comparison, and there's a possibility of taking Unicode input, there may be a more robust answer. Consider instead using fc (fold case) if you are using Perl 5.16.x, or Unicode::CaseFold with earlier Perl versions to obtain a case-neutral basis for comparison.
There are some good articles on the topic, but the following are probably among the best resources available:
I'm probably over-thinking it though. It's possible you live in an ASCII-only world, and simply need lower case for some reason other than case-neutral comparisons, in which case, this post is irrelevant. ;)