Oops.

Interesting, also, that the XS version does short-circuit. I looked at the source and thought I could see that, but I don't know XS so to be sure, I tested. And lo:

$ perl -MList::Util=first -le'@_=(0)x100; first { ++$a; $_ == 0 } @_; 
+print $a'
1
[download]

Hmm.