After my perlbug submission went in, I've been on the edge of discussions from perl5-porters, and the consensus at the moment is that those tests are probably not any good and that I have a working 5.6.1 on my system.
Further update: On being contacted by the owner of the Math::BigInt module, I was asked to run the tests from the brand spanking new (okay, it's eight whole days old) version 1.49 and (wait for it) they all passed.
Final update: I've installed Math::BigInt version 1.49 and everything is happy. As of the time of this writing, unless you are on a Compaq iPAQ running 5.6.1, consider upgrading.
They laughed at Joan of Arc, but she went right ahead and built it. --Gracie Allen