well, I should have tried bigrat before I replied...

If I just 'use bigrat' I get this during make test:

Deep recursion on subroutine "Math::BigInt::bsub"

And then, after a very long time:

Out of memory!

It's possible some Math::* module will help but I don't know which one since I don't understand the underlying problem.

One question I have is why uselongdouble gives less accurate results in this case?

And also, shouldn't all versions of Perl return the same result for a relatively simple calculation?