Here's the result of a simple grep:

Using grep means that you are removing some lines, while your program does not. You might very well have removed the line that causes the problem. You repeated the assumption I suggested you verify.

if ($log_line lt $earliest) {
   print("DEBUG -- last:\n");
   print($log_line);
   last;
}
[download]

I bet you won't find a timestamp at the begining of that line.