pretty amazing, that "optimization":

$ perl -MO=Deparse -e '$a = () = split(" ",$_,0)'
$a = () = split(" ", $_, 1);
[download]

BUT:

$ perl -MO=Deparse -e '$a = () = split(" ",$_,100000)'
$a = () = split(" ", $_, 100000);
[download]

so it's not completely impossible to use it with split, just limited to a fixed maximum number of fields in the split.
It's a bit unsuspected (at least for me) though, that an explicit  split(" ",$_,0) was 'optimized' also.

Btw - just out of curiosity put this version in the benchmark also - and it's faster than the regexp version, but still slower than  $n = @{[ split ]};

I.