Slow down, you're going off in all directions here. The third argument is specific to the CGI::Session driver you choose. It is documented there (e.g. in CG::Session::MySQL), although I would agree that the documentation for it is not very clear.

CGI::Session has nothing at all to do with Class::DBI. If you want to store an object in the session that gets returned from CGI::Session->new(), you call the param() method on that object. Class::DBI objects are already stored in the database so there is no need to store them in the session too.