I've only viewed the source of a few modules, but I've thought that the Date:: modules to be relatively short/easy to understand. However, I've not attempted to view any them using a "the way to code in perl" filter, so whether any of them are well written, I can't comment on.

I think any module you use effectively a lot is something you can learn from, provided you keep a healthy "grain of salt" attitude toward any given incantation. Whether a given incantation is great or not usually depends on what kind of efficiency filter you happen to be viewing it through.

-Scott