I think your earlier idea of the problem being with my installation of Time::HiRes is correct. I just reinstalled it and now in my orginal test I'm getting 0 matches. I also re-ran the piece of code from jethro and where previously I was only getting different times when the loop was running 9000 times or more I'm now getting new times with as little as 5 or 6 loops.

I have no idea how to managed to figure this out but thanks for your help