ikegami pointed out all the positives of mod_perl over plain cgi, and justifiably so. Keep in mind, however, that the very mechanism that makes mod_perl powerful (keep the perl interpreter and your program in memory) makes it also break many plain cgi scripts, especially when the latter depend on getting their detritus cleaned up simply because of die-ing at the end of their run. In other words, the cgi script starts with the user's invocation, and end at the end (!), while mod_perl scripts don't really end. If you have variables that depend on being "fresh" on every invocation, you will be in trouble.

I haven't done any work with mod_perl thus far (to speak of), and I guess I have missed its power. However, I haven't really needed its power thus far. The book that Arunbear suggests is going to be very helpful in moving from cgi to mod_perl.