mod_perl integrates perl with the Apache2 web server. It can be used to
allow the users to use Perll for just about anything Apache-related,
including <Perl> sections in the config files and the famous
Apache::Registry module for caching compiled scripts. 

It can produce anywhere from a 400% to 2000% speed increase on sites using
perl scripts, and is used on many large script-based web sites - for
example, http://slashdot.org.
