http://perl.apache.org/: The mod_perl home page, with downloads, documentation, sample modules, insights and stories describing mod_perl successes. This site also tells how to join the mod_perl e-mail list, where developers trade ideas, enhancement requests and bug reports.
In particular, you might want to look at the mod_perl guide which includes installation and debugging information, found at http://perl.apache.org/guide/.
Finally, go to http://www.modperl.com/, a site established by the authors of the coming Apache modules book. This book describes how to write modules for Apache in C and Perl, and includes a great deal of information about mod_perl in particular. Even if the book has not yet been published by the time you read this, the site contains ideas and draft chapters documenting the mod_perl API better than the on-line documentation.
