Try this at the command line:
perl -MCPAN -e 'install Date::Calc'
If this works, it will install a module for date calculations on your system. You can name any module you wish instead of Date::Calc.
If this doesn't work, then I think you have a strange distribution of perl. If you are on windows though, you probably have activeperl from activestate, and there is a PPM (perl package manager) for that. I'm not sure on it's details though - since thank-jobs I am not using windows.
--jim