It's used here actually:

my ($left,$middle,$right) = $sequence =~ m/(\w{$_})?(\w{1})?(\w*)/;
                                                ^
[download]

The result for 0..6 would be:

              A  CTGACN
      A       C   TGACN
     AC       T    GACN
    ACT       G     ACN
   ACTG       A      CN
  ACTGA       C       N
 ACTGAC       N
[download]