split() in scalar context (update: void context is a special type of scalar context) assigns to @_. That's documented. Chances are you can use something other than split() if you just want a field-count.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;