if ($file_contents_changed) {
   my $bak = $fname . $$ . '.tmp';
   -e $bak and (unlink($bak) or die "unlink $bak: $!");
   open(my $fh, '>'.$bak) or die "create $bak: $!";
   print $fh @lines or die "writing $bak: $!";
   close($fh) or die "close $bak: $!";
   defined(my $mode = (stat($fname))[2]) or die "stat $fname: $!";
   chmod($mode, $bak) or die "chmod $bak: $!";
   $^O eq 'MSWin32' && ! -w _ and
      (chmod($mode|0200, $fname) or die "chmod $fname: $!");
   rename($bak, $fname) or die "rename $bak $fname: $!";
}