my (@keys)= keys %DNA_sequence_count;
foreach my $key (@keys) {
  for ($i=$sequencelength-1; $i; --$i) {
    $DNA_sequence_count{substr($key, 0, $i}+= $DNA_sequence_count{$key};
  }
}
# correction again thanks to ikegami!
for ($i=$sequencelength-1; $i; --$i) {
  # second correction thanks to sauoq
  for ($j=$i; $j<$sequencelength; ++$j) {
    ++$DNA_sequence_count{substr($allbuffered, -$j, $i)};
  }
}