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

##</code><code>##

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