I addressed this in chapter 7 of my book - Writing Perl Modules for CPAN. You can download it for free.

Interestingly, the two modules mentioned above - DBI and CGI - are both on my list of great CPAN modules but I don't consider either to be "good reads" for learning about Perl module design. They both contain rather unusual abuses of Perl's OO system that shouldn't be replicated without due cause.

-sam