my $length = length $string;
my %seen;
for my $i (0 .. $length - 1) {
  $seen{ substr($string, $i, 1) } ++;
  $seen{ substr($string, $i, 2) } ++ if $i < $length - 1;
  $seen{ substr($string, $i, 3) } ++ if $i < $length - 2;
}