There are several done in PHP, including LOCal. It is implemented using PHP and MySQL. Porting it to perl would be relatively simple.

It's not entirely difficult to write one but when it comes down to feature implmentation it can get big quickly (need for user auth, users only modifying their own entries, admin functions, editing functions, aggregate functions, multiple display modes, as well as all the idiot-proofed form handling). The display modes that users might like to see are date-oriented (for those wanting to reserve "Next Tuesday") as well as summary list (show me everything reserved) including a grid view for a particular day. Aggregate handling features might include a facility for reserving regular usage "Bob runs a job every other Tuesday this month". The good news is, that the if statements that do automagic boundary checking to prevent double booking is all of 2 lines.

ModPerl would probably be preferred over straight CGI because of the extensive form handling and html rendering required. Don't worry about re-inventing the wheel. I looked for ages for 3rd party OSS solutions and didn't find anything satisfactory, so I implemented my own. The advantages of rolling your own is because you maintain code base control, as well as being able to ubercustomize the application. Currently I regret the lack of code to post, because none of it is in perl :( (i'm a newbie to this forum) so you'll have to wait while I port some stuff over.