The usual recommendations include Date::Calc and company.

Depending on what your system is, ppm (for ActiveState) or CPAN for others will let you get and use the module.

You'll also want to note that localtime does not count months as "1"-based, as you appear to be doing (pardon asked if I'm wrong):

From perdoc -f localtime

...$mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating December. ....

Just as an aside, you'll find that search and/or SuperSearch right here at the Monastery will be very helpful.