• Take a look at CPAN: a simple search for HTML::Calendar return 4 results: don't reinvent wheels!
• Again if you got to manipulate dates a search on the friendly CPAN return lots of results: take a look at Date::Calc, Date::Manip and Time::Piece to start.
• If you use CGI use it indeed! It's really comfortable to build tables and lots of other stuff (from writing headers to process query string, etc.)

$|=$_="1g2i1u1l2i4e2n0k",map{print"\7",chop;select$,,$,,$,,$_/7}m{..}g