Since you can only action one file in the <form>, whichever one you choose would have to send the form data on to the other.
I presume either one can be used as a form action script, ergo they're accessible via an http request. So in the action script construct such a request with the form data inside and send it to the other via the web server.
How you do this in Perl is debatable (tmtowtdi), and also in PHP. But in the latter case I'd consider using the cURL functions, so's I don't have to worry my pretty little head about composing appropriate headers and formatting the data as per CGI standard.
There may be other solutions in PHP, using the system(), exec() functions or the `` operator, and no doubt there are a number in Perl, if there are alternate ways than a CGI request to supply the data (have the first script to write a temporary file for the other script to read and then delete?). But the final answer is that one of the scripts has to call the other with the form data.