You said "delete" in your problem description, but I don't see any call to unlink. Did you mean you have problems renaming the file? Why don't you check the return value of rename and check $! if an error is reported?

if ($linenumber) {
   rename $logfile, $logfileOrig
      or die("Can't replace log file: $!\n");
}
[download]

Note that $currentline can be replaced with $..