$s = 'aaaabccbaxxxcbddadcc';;

++$h{ $1 } while $s =~ m[(?=([abcd]{3})).]g;;

pp \%h;;
{
  aaa => 2,
  aab => 1,
  abc => 1,
  adc => 1,
  bcc => 1,
  bdd => 1,
  cba => 1,
  cbd => 1,
  ccb => 1,
  dad => 1,
  dcc => 1,
  dda => 1,
}