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  $rgb[16] = 7360;  # Overwrite 3689 with 7360
  $rgb[22] = 4912;  # 2462 -> 4912

  open(my $outfile, '>:raw', $hfn)
    or die "Couldn't open $hfn: $!\n"; # Overwrites existing file
  print $outfile pack("S*", @rgb);
  close($outfile)
    or die "Couldn't close $hfn: $!\n";

  open(my $fh, '+<:raw', $hfn)
    or die "Couldn't open $hfn: $!\n";
...
  print $fh pack('S', 4912);
  close($fh)
    or die "Couldn't close $hfn: $!\n";