I program Perl CGI every day. What often works for me is to use no static HTML pages at all, and to generate all HTML dynamically.

If Form A calls Form B calls Form C, then have the form handler for Form A do its processing and then generate Form B before it exits. Similarly, the form handler for Form B should do its processing and then generate Form C before it exits.

I would not combine the form handlers for two dissimilar pages in the same script.

However, if two pages are similar then I call them both using the same form handler. I use CGI.pm's referer() (sic, only three r's in the CGI.pm spelling) function to see which page called the form, and then have the form modify its processing accordingly.