write them to a file on the users computer
Otherwise, try to follow a tutorial. Handling forms with Perl is a completely basic topic, covered a thousand times in different places, and written better than I ever could. Also, it requires a lot of details, like server configuration, so it's too long to simply spell out in a simple answer like this.
Try this one (though I don't have experience with it) www.cgi101.com/learn/
I personally prefer to work with a MVC framework named Catalyst. It's easy to start with. If you're interested, start here: http://search.cpan.org/perldoc?Catalyst::Manual::Tutorial