IMO, the Perl literature is relatively thin on matters of module/class design and organization. A typical response to this observation is to point to CPAN while saying the words "Use the Source, Luke!" Unfortunately, CPAN modules are often quite difficult to read. This may be because the module is badly written, or because it is doing some very cutting edge stuff that only the most advanced Perl programmers can follow. Or perhaps because it is too huge to be comprehensible by the uninitiated.
What CPAN modules stand out in your experience for their readability and "didactic value"?