mod_perl is a module that embeds the PerlLanguage interpreter into the ApacheHttpd web server. This improves performance of Perl CGI scripts and exposes the full Apache ApplicationProgrammingInterface to Perl modules. It also simplifies maintaining session state because the Perl interpreter remains running, so data can be stored within the interpreter.
There are similar modules for other languages, such as ModPython and ModRuby?.
More info at http://perl.apache.org/
Another way to accomplish the same benefits (performance and session state) is to use FastCgi, which doesn't couple so tightly to the web server.