my %Y;
while (<>) {
  ...
  my ($y) = ... ;
  next unless $y;
  
  # Add the scalar number of elements in @ARGV to the hash
  # Always add at least 1 to the hash even if @ARGV contains
  # no elements
  # the value $y is meaningless and plays no role
  
  if(@ARGV) {
      # next is always executed regardless of $y eq $_
      foreach (@ARGV) { $Y{$y}++,next if $y eq $_;  }
  } else { $Y{$y}++ }
}