... and this approach can easily be extended to make n dynamic by interpolating the width counts in the unpack template string.

>perl -wMstrict -le
"my $text = 'howdoidoit';
 ;;
 my $n    = 3;
 my $back = $n - 1;
 ;;
 my @unpacked = unpack qq{(a$n X$back)*}, $text;
 my %count;
 $count{$_}++ for @unpacked[0 .. $#unpacked - $back];
 ;;
 use Data::Dumper;
 print Dumper \%count;
"
$VAR1 = {
          'wdo' => 1,
          'ido' => 1,
          'owd' => 1,
          'how' => 1,
          'oid' => 1,
          'oit' => 1,
          'doi' => 2
        };
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