What should I read to learn building modules over existing ones?

A good idea is to assume you're not the first to have this question (whatever it is), and to search for the answer

How to install CPAN modules into ActivePerl, A Guide to Installing Modules, ... Yes, even you can use CPAN, A Guide to Installing Modules, Top 11 (GOOD) reasons not to use someone else's Modules, Top Seven (Bad) Reasons Not To Use Modules

perlnewmod, perlmodlib, ...

Tutorials: Perl documentation documentation, Searching Perl Documentation, How to RTFM