perl spends a lot of its time doing hash lookups
True, but my (unstated) thought was that such 'hash quality' check would only need to be done at STORE time, not FETCH time. As such, the costs would be a significantly smaller proportion of the overall operation than with the FETCH, as it would be along side the memory allocation, bucket shuffling etc.?
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller