sub filter4 {

    my ($ar_sequences,  # ref. to array of sequences to be filtered
        ) = @_;

    # eliminate duplicate sequences of equal length.
    # a sequence may still exist as a substring in a longer sequence.
    my @from_uniq = uniq @$ar_sequences;

    # separator string.  must be distinct from anything in a sequence.
    my $sep = ':';

    # string of unique sequences, sorted shortest to longest.
    my $uniq_short_to_long =
        join $sep,
        sort { length($a) <=> length($b) }
        @from_uniq
        ;

    # joined string must end with a separator string.
    $uniq_short_to_long .= $sep;

    # some convenience regexes.
    my $base = qr{ [^\Q$sep\E] }xms;
       $sep  = qr{   \Q$sep\E  }xms;  # convert to regex object

    # build index of offsets of first position beyond each length.
    my %offset;

    LENGTH_GROUP:
    while ($uniq_short_to_long =~ m{ \G ($base+) $sep }xmsg) {

      # build regex for sequences of this length.
      my $n_bases = length $1;
      my $n_seq   = qr{ (?:$base){$n_bases} $sep }xms;

      # find, save offset of 1st longer seq after these sequences.
      $uniq_short_to_long =~ m{ \G $n_seq* }xmsg;
      $offset{ $n_bases } = pos $uniq_short_to_long;

      }  # end while LENGTH_GROUP

    # keep all sequences NOT substrings of any LONGER sequence.
    return
        grep {
            $[ > index $uniq_short_to_long, $_, $offset{length $_};
            }
        @from_uniq
        ;

    }  # end sub filter4()