my $num;

while (<FILE>) {
   if ($. % 3 == 1) {
      $num = sprintf("%02d", rand(100));
      substr($_, 0, 2, $num);
   } else {
      substr($_, 1, 2, $num);
      substr($_, -3, 2, sprintf("%02d", rand(100)));
   }

   print;
}