I think the other answers here were a little quick on the trigger. You can't do everything in an htaccess file, but you can do a lot. The details (for mod_perl 1.x) are covered here.

As for your second question, you don't even have to do that much to use mod_perl. If you set up Apache::Registry or Apache::PerlRun, you can run existing CGI scripts without changes. You will run into problems though if they are sloppy scripts that don't pay attention to issues like variable scoping. This is all covered in The Guide.