You call your Perl script to process the form in the form tag attribute "action". It's the same on any form. You can either code it by hand, or use the form editor to set it up.
<form name="NameOfYourForm" method="post" action="path to your script/
But after reading further in your comments I don't think that is your problem, is it? Your scripts work correctly when you upload them to your server, but you want to test them prior to uploading them, right? If I recall correctly, FrontPage won't run code unless you have a server installed on your test PC. Years ago FrontPage came with some sort of mini-personal-web-server thing that you could set up to allow scripts to be run--I don't know if that's still the case. I remember that I had to upload my code to the server to test it out. But, that was years ago and many FP versions back. I don't know what you might have available if you're using a new version of FP. If it doesn't come with a server you can configure, you might try installing and using Apache on your PC. Or, get a copy of OptiPerl. It's a shareware development tool that will let you test server-based scripts.