Well there are various things on the net.

Looking for something that has:
- graphic interface
HTML::Calendar::Monthly /Tk
- email reminders
Remind::Client
- multiple users / calendars (using already existing internally created rights system)
Authen::Users
- over all calendar - save data in mySQL db
DBD::MySQL or DBIx::Class
- can programmatically add to calendar or db DBIx::Class

See if you can come with something cool.