If you really, really want Apache to allow POSTs to .shtml files to pass through to SSI-embedded CGI scripts, then you still have two options left open:

1. Do it yourself. Download an Apache source tarball, and start hacking on the source until it works the way you want it to. Depending on your C skills, this might take 1-10 days.
   
   
2. Pay someone else to do it. If you're unable or unwilling to hack Apache source yourself, consider sponsoring a project at Collab.net. Someone might take up the challenge for a few thousand dollars.

If you're willing to retreat a bit on your desire to have Apache do something that it doesn't do out-of-the-box, then you have a third option -- one hinted at in at least one reply above: Modify your CGI scripts. Before creating a CGI object, force-feed %ENV with the variables that CGI.pm expects to find. You'll find what you need in the enviroment.