The Y! Calendar is one of the few Y! properties that is not (yet) accessible through an API so either you'll have to reverse engineer the Intellisync program, or go for scraping *shrug*.

If you do succeed, please do post your work at CPAN (or here), for I believe a lot of people would love to manipulate their Y! calendar through Perl. Maybe then I'll switch back (currently using 30boxes.com, which IMHO is a lot nicer).