I finally figured it out today!
The problem was in the test code. The tests simulate a CGI environment by setting an envvar (HTTP_ACCEPT_ENCODING=gzip) and calling an external, very simple CGI program via backticks. The problem is that some smoke systems don't pass envvars (probably due to taint). So, the CGI always ran in non-gzip mode. The fix was simple.
Even though this test failure was spurious and was never reported by anyone except a smoke tester, I'm relieved to have fixed it finally. As always, I'm grateful for the patience and persistence of the smoke testing community! Perl would be nothing without its CPAN support community.
If this new release is good with the smoke testers, I'm going to push out a 1.00 release at long last.