mod_perl is a module for the Apache web server. It embeds a Perl interpreter into the Apache server itself, so that dynamic content produced by Perl scripts can be served in response to incoming requests, without the significant overhead of re-launching the Perl interpreter for each request.
This improves overall performance of perl based application.
This is an exhaustive list of various techniques you might want to use to get the most performance possible out of your mod_perl server: configuration, coding, memory use, and more.
This is one of the best resources on tuning perl based app servers, is the mod_perl performance tuning documentation. If you are looking for mod_perl Apache optimization this is a must read for you :)