The first version does not allow for empty @ARGV; both versions do not allow for the possibility that @ARGV may have repeated elements.

I think a short form that preserves all the current behaviour could look like this:

  ...
  next unless $y;
  $Y{$y} += @ARGV ? (grep $y eq $_, @ARGV) : 1;
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It would, though, be preferable to have some clue about what the code is intended to achieve: it is always possible that the behaviour on repeated elements is unintentional or irrelevant.