There are a number of component systems either built around mod_perl or are widely used in the mod_perl community

HTML::EmbPerl

An early development in the mod_perl community, HTML::EmbPerl expresses logic top to bottom embedded within HTML. While it's a very mature system, it seems to have been playing catch-up with regard to caching and its object model.

Apache::ASP

This implements request, response, session and other ASP objects using Perl. If the ASP API appeals to you and you don't want to use Microsoft or ChiliSoft technologies, Apache::ASP is the way to go.

HTML::Mason

Mason components can be HTML (or XML or FooML for that matter), Perl code or a combination of them. Components may generate output or they may be stand alone routines that accept data inputs and return data outputs. The object cache stores components as Perl code so evaluation from the cache is very fast.

AxKit

An XML & Sablotron (XSLT Processor) based system, it's pretty new, so I haven't looked at it closely yet!

There are links to these and other mod_perl technologies on the mod_perl site, http://perl.apache.org/.