I assume you are interested in the number of calendar weeks a month straddles rather than a simplified "February has 28 days so only 4 weeks in February..."

I don't really know of a module that gives that answer but Date::Calc has a Week_of_Year function which you could utilise (use it on first and last day of the month , then subtract the weeknumbers and add one), which should give you the number you're looking for...