I'm currently writing a module, Test::WWW::Mechanize::HSSCGI, which will dry-test a HTTP::Server::Simple::CGI server without needing network and/or sockets. While looking for it, I found Test::WWW::Mechanize::CGI, which seems to be just what you need, a WWW::Mechanize that knows how to talk to a CGI backend without having to have a webserver. So maybe you can use that, or the methods employed by it.