"Just the FAQs: Coping with Scoping," an article by Mark-Jason Dominus about how Perl handles variables and namespaces, and the difference between use vars( ) and my( ): http://www.plover.com/~mjd/perl/FAQs/Namespaces.html.
It's important to know how to perform exception handling in Perl code. Exception handling is a general Perl technique; it's not mod_perl-specific. Further information is available in the documentation for the following modules:
Error.pm, by Graham Barr.
Exception::Class and Devel::StackTrace, by Dave Rolsky.
Try.pm, by Tony Olekshy, available at http://www.avrasoft.com/perl6/try6-ref5.txt.
There is also a great deal of information concerning error handling in the mod_perl online documentation (e.g., http://perl.apache.org/docs/general/perl_reference/perl_reference.html).
This page describes the mechanics of creating, compiling, releasing, and maintaining Perl modules, which any mod_perl developer planning on sharing code with others will find useful.