You still use CGI even when you use mod_perl. CGI is the Common Gateway Interface, the way Web clients pass data such as form entries to an application on a server.

Actually, no. CGI is an interface from servers to external applications. Clients have nothing to do with it. And when using mod_perl or mod_php, you are not using CGI. These are apache modules. They run directly in the apache process. In summary: CGI is external, modules are internal.

[If you don't believe me, feel free to consult the CGI Intro linked from W3's CGI page. Notice the first sentence of that intro says "The Common Gateway Interface (CGI) is a standard for interfacing external applications with information servers, such as HTTP or Web servers." The emphasis is mine.]