You can call a CGI program from within another CGI program (take a look at do), but you usually don't want to - since normal CGI programs set up a response and parse input so you have to be really careful to not entangle the output and you'd be parsing the input twice for no good reason.

IMO, if you're going this route, you'd be much better off using CGI::Application combined with CGI::Application::Session. That'll do 3 things: it'll set up a session automatically, and it'll help you do multiple actions in a single script in a reasonably consistent and easy to follow manner, and it'll make it very easy to use templates.