One remark: the regex engine is fast, sub invocations are slow. To reduce the overhead of sub invocation, I'd replicate @x a few hundred or thousand times.

I was hoping that regex speed would improve, or at least be statistically similar, from release to release. In some of these cases, however, that does not seem to be the case. I then went and tried all the versions I have and got some widely varying numbers. Is this what others would expect?

As far as I remember, many changes to the regex engine improved its sanity with respect to Unicode strings - mostly their semantics, speed suffered a bit from time to time.

So it's not too surprising that some of them got slower.

In case of the "single" benchmark, it might be worth to clone the perl git repo, and use git bisect to find out which changeset it was that slowed it down that much - if it was a single one. Then tell p5p about your findings, maybe the performance loss can be tackled.