The extension .html no matter, is just the configuration of your webserver that link .cgi , .pl ... to the perl interpreter ( or mod_perl ). You can arbritrary choose whatever extension you want. Of course, this supose you have access to the configuration files.
Mason is supposed to be used for embedding perl in html. I dont know any details of it, but I believe its meant to be used with mod_perl and Apache. Mason v1 was recently released. www.masonhq.com Perhaps someone could enlighten us more about this?