I wrote an eXtended Test (xt/verify_strong.t) for Math::Primality which verifies that it correctly identified all strong pseudoprimes less than 1e15. That is 1,000,000,000,000,000 ! There end up being 419489 such numbers, so I thought nothing of writing
use Test::More tests => 419489;
Needless to say, I was a bit worried to see my test process constantly eating RAM, ending up utilizing about 650MB of RAM before exiting. I thought that maybe there was a memory leak in Math::GMPz, which Math::Primality uses to access the GNU MultiPrecision (GMP) library. But after making this small patch to roll my own ok() method, my memory usage plummeted to about 50MB, over a 90% memory usage decrease!
The very next day I see garu talking about the very same issue. Evidently it is "working as designed" according to Schwern and it comes down to Test::Builder saving the history/output of every test, which is not currently configurable. This is one of the many things that Test::Builder2 will hopefully solve.