I finally figured out that this works:
$Apache::TestRequest::RedirectOK = 0;
Again, more docs would be welcome.
well, it's not more docs, but whenever I'm in search of some bit of functionality I check the tests in the perl-framework/t/modules directory, which you'll find if you checkout the httpd-test project or view the CVS as linked to from http://httpd.apache.org/test/
your solution, for instance, is used in alias.t:
## make LWP not follow the redirect since we ## are just interested in the return code. local $Apache::TestRequest::RedirectOK = 0;
HTH
--Geoff