open(my $fh, '+<:raw', $hfn)
    or die "Couldn't open $hfn: $!\n";
  seek($fh, 2*16, 0); # Double up 'cause 'seek' wants bytes not short ints
  print $fh pack('S', 7360);
  seek($fh, 2*22, 0);
  print $fh pack('S', 4912);
  close($fh)
    or die "Couldn't close $hfn: $!\n";