If you run your code under warnings, Perl complains about Odd number of elements in hash assignment. Nothing unexpected. But if you benchmark just one of the versions at a time, there's an interesting nugget to be found.

The list bench is predictable and boring. The screen fills with warnings emitted at a constant rate.

But the shift benchmark exhibits a rather inexplicable pattern. The fact that the warnings eventually stop is easy to understand: modifying the caller's @_ is a sideeffect that persists across iterations. What is really strange is that the warnings are emitted at progressively slower rate.

Why? I don't know. I clawed around in the bowels of Benchmark.pm briefly but didn't find it very pleasant to read, so I gave up. Maybe someone more motivated than me wants to pick up this riddle.