The Apache/Perl module list is here for the same reason as the main Perl module list, to change duplication of effort into cooperation and help to save the world! Apache/Perl modules are somewhat different from the many others found on CPAN. Most Perl modules are "building blocks", developers use them to build applications or even more building blocks. It just so happens that Apache/Perl modules are encapsulted inside a namespace and .pm file. However, this does not make them all building blocks, many are just like apache modules written in C, you install, edit the server configuration files and you're done. Before you start a new module, consider this: Apache/Perl modules are useless outside of mod_perl and Apache. Do your best to design such that your Apache::* module will make good use of the building blocks found on CPAN, putting together new building blocks where appropriate and simply glue them together with the Apache API, keeping the Apache::* modules very small.
These modules will also serve as good examples, showing you where modules can step in during a stage of a request with Perl*Handler callbacks. You'll also see how and when to use the Apache API methods.