DB<30> $str = join "", a..l

  DB<31> @res=()

  DB<32> for my $start (0..2) { pos($str) =$start; push @res, $str =~ m/(.{3})/g }

  DB<33> x @res
0  'abc'
1  'def'
2  'ghi'
3  'jkl'
4  'bcd'
5  'efg'
6  'hij'
7  'cde'
8  'fgh'
9  'ijk'