Probably the best source of advice is ... existing modules on CPAN.

That's a silly advice. Anyone can upload any crap on CPAN, and many people do. There are modules with good POD on CPAN, but there are also modules with shitty or hardly any POD on CPAN. The existence of a module on CPAN does not imply anything about its quality (code nor documentation wise). Just pointing to CPAN isn't very helpful in this case.