You should consider using HTML::CalendarMonthSimple, which I've used before for Calendar-WWW-Stuff and which works really create and is VERY_simple to use. There are even other modules for that if you need something more specific. Why should you want to rewrite all that stuff that is already there? And if you want to for some reason, have a look at Date::Calc to get your days_per_months etc.
When you use such modules you probably will also have less problems in your code ;)

giant