The main point of mod_perl is to already have a copy of perl running. This avoids load time for each invocation. Trade-off against vanilla cgi is slight and improvements in response time can be large. It is not a choice of 'cgi' or 'mod_perl', normally you will use both. To quote a quote:

mod_perl is more than CGI scripting on steroids. It is a whole new way to create dynamic content by utilizing the full power of the Apache web server to create stateful sessions, customized user authentication systems, smart proxies and much more. Yet, magically, your old CGI scripts will continue to work and work very fast indeed. With mod_perl you give up nothing and gain so much! http://en.wikipedia.org/wiki/Mod_perl

There are further(many) advanced uses of mod_perl— little of which is particularly down to earth. After all, O'Reilly doesn't publish books about simple things for the most part; but you might toss these on your developer(s) desk(s):

• Practical mod_perl by Stas Beckman and Eric Cholet. O'Reilly<\li>
• Writing Apache Modules with Perl and C by Lincoln Stein and Doug MacEachern. O'Reilly
• mod_perl: Developer's Cookbook by Geoffrey Young, Paul Lindner and Randy Kobes. Sams.