# Slurp file $fname into array @lines.
open(my $fh, $fname) or die "open '$fname': $!";
my @lines = <$fh>;
close($fh);

# Mangle @lines as appropriate ...

if ($file_contents_changed) {
   my $bak = $fname . $$ . '.tmp';
   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: $!";
   -w _ or (chmod($mode|0200, $fname) or die "chmod $fname: $!");
   rename($bak, $fname) or die "rename $bak $fname: $!";
   chmod($mode, $fname) or die "chmod $fname: $!";
}