I increased $max to 112 and it was consistently doing what you described. So I started reducing $max and it appears to be stable at 107. At 108, it displayed the results 14 out of 20 attempts. At 109, out of 20 attempts: 11 displayed, 6 didn't and 3 crashed. At 110, out of 20 attempts: 5 displayed, 10 didn't, 4 crashed with no error and 1 crashed with "Free to wrong pool 222ca8 not 5." At 111, out of 20 attempts: 3 displayed, 4 didn't, 12 crashed with no error and 1 crashed with "Free to wrong pool 222ca8 not 5." At 112, out of 20 attempts: 2 displayed, 3 didn't and 15 crashed with no error.

Update: I guess I can stop digging through perlguts, perlapi and perlcall now that you fixed it.