# check for semaphore file
  my $data_dir = "C:/data";
  my $semaphore = "$data_dir/inprocess.txt";
  die "another process is in progress" if -f $semaphore;

  # create the semaphore file
  open SEMAPHORE, ">$semaphore" or die "Can not create semaphore";
  close SEMAPHORE;

  # do the processing...

  # delete the semaphore file at the end
  # you could put this bit in the END block.
  unlink $semaphore;

##</code><code>##

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