Perhaps you don't know that what the $ prototype does is place scalar(...) around the given argument to the function.

sub foo ($$) {
  print "<@_>\n";
}

foo(@this, %that);  # foo(scalar(@this), scalar(%that));
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An array evaluated in scalar context returns its size.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;