I'm not sure if you can send username and password via a form using only CGI.pm, but there are a couple of modules on CPAN which claim they can do it under mod_perl (supposedly they interact with the built-in perl interpreter in some magic way which I don't understand yet). So, you may want to check out Apache::AuthCookie and Apache::AuthTicket.

When I designed my first (and only as of yet) interactive site I stuck to basic authentication but soon discovered its limitations. For example, there's no obvious way to create 'guest' accounts, and adding new users is a hassle.

This assumes you are running under Apache of course, which I now realise you never said you did.

Cheers, --Moodster