#!/usr/bin/perl -w
   use strict;
   
   open (F, "+<",$ARGV[0]);
   my $out = "";
   while(<F>){
       chomp;
       $out .= $_."\tadd sth\n" if($.%2 == 0);
   }
  seek(F,0,0);
  print F $out;
  close(F);