You don't really need to understand OOP to use CGI::App. You just need the basics of how to call methods on an object. That much is easy to pick up from CGI::App examples. (Of course, understanding OOP would only help, and would become essential if you wanted to extend CGI::App.)

What weierophinney said: use CGI::Simple instead of CGI for all of the parameter processing goodness without the bulk of the HTML creation code. Note that CGI::Session doesn't subclass CGI, but it does do an if isa CGI test, so it would need tweaking to use it with CGI::Simple. But if you're trying to avoid external modules, realize that CGI::Session uses a bunch of 'em.