Just wrote up a small pseudocode for a previous post here.
 [sourcecode type="php"]
open (IF, "infile.txt");
 open (OF, "outfile.txt");
   while (my $line = <IF>) {
     $count++;
     if ($count == 10) {
       # Modify
       print OF "somethingelse\r\n";
     } else {
       # Don't modify
       print OF $line;
     }
   }
 close(OF);
 close(IF);
[/sourcecode]