I found your description rather confusing. You are not interested in substrings, rather in substrings that do not find repeated characters. (I take it that UCS means Unique Character Substrings?) For comparison here is another approach based on recursive splits of the string rather than repeated calls to index. I have no idea how it compares for efficiency. 
sub tilly_UCS {
  my $str = shift;
  # Try all cuts.  Those that don't fall in 2 are repeats
  foreach my $char (split //, $str) {
    my @cut = split /\Q$char\E/, $str, -1;
    if (2 != @cut) {
      my @rejoined = map "$cut[$_-1]$char$cut[$_]", 1..$#cut;
      @rejoined = sort {length $b <=> $a} @rejoined;
      my @unique = tilly_UCS(shift @rejoined);
      foreach my $str (@rejoined) {
        if (length($str) < length($unique[0])) {
          last; # Avoid useless work, cannot improve
        }
        my @found = tilly_UCS($str);
        if (length($found[0]) > length($unique[0])) {
          @unique = @found;
        }
        elsif (length($found[0]) == length($unique[0])) {
          push @unique, @found;
        }
      }
      return @unique;
    }
  }
  # No repeats
  return $str;
}

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BTW those that don't know what the third argument to shift did should look it up. You will avoid a commmon misunderstanding that can lead to bugs.

In reply to Re (tilly) 1: Unique-Character Substring by tilly
in thread Unique-Character Substring by japhy

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