For Perl up to and including 5.10, calling split in scalar or void context had the side-effect of setting @_.
This mis-feature was removed in Perl 5.12.
That was great news for most Perl programmers because this side-effect "feature" was unnecessary, an ugly wart.
It was sad news for golfers though because side-effects are frequently useful in golf.
In fact, I recently experienced a similar split annoyance in Compression in Golf: Part III, as described in the "Fun with split" section in that node.
Simply changing @_ to @z say should fix thospel's original solution at the cost of three strokes:
I expect you could do better than that, though I haven't tried.