Mod-Perl is an add-on for the Apache web server software, which makes it possible to use the Perl language to add new features for the Apache server, and to increase the speed of Perl applications by as much as 30 times. In other words mod_perl is a perl interpreter which is built into the Apache web server. Mod_perl brings together Apache and Perl, two of the most powerful and mature technologies available to the online world today.
Mod_perl gives the Apache server the ability to execute Perl code directly without having to call a CGI which reduces the startup burden associated with running Perl scripts from the web server. It also allows variables to persist across multiple requests, and lets user authentication be extended to anything that can be written in Perl i.e authenticate web access against an SMB or RADIUS server without having to write an Apache module to do the job.