This thread on p5p explains the problem. In short...

For some reason the optimiser has decided that there are common elements
on both sides, and that therefore a temporary copy of each element must be
made. This is to handle cases like:

    ($a,$b) = ($b,$a);

but is now slowing down the normal case.

It was tracked back to this patch. Now that it's been nailed down there's a very good chance it'll be fixed in 5.10.1.