Damian Conway, the original author of the code in question, says:

That's so baroque, I'm sure it must originally have been intentional, though it's now clearly vestigial.

An earlier version probably did something like:

        foreach (@{[@{$_[0]}]}) {...}
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where the inline copy was essential, presumably to prevent $_ iterating the original arguments. And then I simply forgot to correct it when I changed the structure.

I've changed it now. Thanks.