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 1234 ABC 100.0.0.0 
4567 DEF 200.0.0.0 .....

 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: $!";