And you want:
IIS >> Apache (mod_rewrite) >> browser

iamnothing, you are correct in the above statement. If the user is redirected to B, I want B to do all of the processing and returning of the results. A should be completely out of the picture once it has performed a redirect to B. B should, however, not be known to the user and report that it is indeed A.

It should be noted that at this point, A is already doing what it is supposed to do. If it receives a request for the page www.example.com/webapp, it redirects to B. Unless I can redirect to B from A and make sure that A doesn't change the user's browser to report that it was redirected to B, I don't need to do anything to A at this point. But I don't think that IIS has that ability, although I may be incorrect.

ALL HAIL BRAK!!!

I'm thinking the same thing, that the problem is not so much in Perl but with the way that IIS performs redirection (unless you are using Perl to perform the redirection).

Have you thought of setting up Apache with a Virtual server name that is the same as A (which shouldn't conflict as long as it doesn't have to pass information back to the real server) should a redirection occur?

I'm installing IIS now. I'll fire up apache on linux and see if I can get it to pretend to be another machine. Won't be able to get to it all today because I have to do some Java training for staff members in about twenty minutes, but I should have some sort of answer (even if it's "hell, I don't know!") by tomorrow some time.