Is it possible to access the fields of a HTML form upon submission within a Perl subroutine?

The question here is: where would that Perl code be running? How would it receive the requests (form submissions) to filter?

So yes, one way to do it would be to set up a proxy web server, and then use the CGI approach you're considering. I.e. the CGI program would receive the requests, parse, manipulate and reassemble the query string, and send it as a new request to the target server.

If setting up a dedicated webserver (like Apache) seems like overkill for that purpose, you might consider using HTTP::Proxy, or some such...