This was the first module I ever wrote for CPAN, and now sits at the core of every large API I create. It has dealt with class trees as large as 200-300 classes, and has been flogged to death and tested in almost every way I can imagine.
I highly recommend this to anyone loading lots of large modules they might not use, or loading very large class heirachies.
Class::Autouse lets you pre-declare classes you intend to use, but doesn't actually load them until the first time you invoke one of their methods. This is a clever hack to avoid run-time loading of modules that you might not actually use.
I haven't used this extensively yet, but the documentation is great and it seems like the author thought of all the important caveats.