or download this
my $file = "$file_path/file.lst";
my $newid = "500.0.0.0"
open MAST, $file or die "Unable to open file.lst: $!";
...
rename($file, $bak) or die "can't rename $file to $bak: $!";
rename($new, $file) or die "can't rename $new to $file: $!";