Check out 17.3.2 of CGI Programming with Perl. There's a section there on mod_perl considerations that's quite concise and helpful. Converting from CGI to mod_perl can sometimes be painful, but once all the kinks have been ironed out, the benefit is really quite nice. As for CGI, I'm still stuck in the stone-ages - I use a variant of ReadParse from an old cgi-lib.pl for most of my stuff.. :-)
  -Adam
P.S. Also be sure so check out the mod_perl docs, though I'm sure you prob already have :-)