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.

In reply to Re: 5:10: Why is this slower than 5.8.8? by schwern
in thread 5:10: Why is this slower than 5.8.8? by jk2addict

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