Recently, there's been a discussion on PerlMonks about how an internal redirect wasn't working. I traced it down to a few lines of mod_cgi.c in the Apache source code. For an internal redirect, a location header must be present, begin with a slash (no hostname) and the status must be 200.
Unfortunately, the standard CGI.pm redirect routine defaults the status to be 302. Thus, you have to remember to write the redirect as
print redirect(-uri => '/new/location', -status => 200);{sigh}
CGIでも Location: と Status: 200 で internal redirect になるって話。だいぶ昔にきいて、そうはならないなーと思って放置してたのですが、status が問題なのですね。