A non-recursive solution (benchmarks about half as fast as powerset with sort, somewhat disappointingly). The idea is that, for example, A in generation 1 becomes AB, AC, AD in generation 2. B in generation 1 becomes BC, BD in generation 2. Each entry in a generation is mapped to each of the characters after its last character to create the next generation.

sub comb {
    my @alphabet = @_;
    my @generation = (['', 0]);
    my @return;
    while (@generation) {
        my @next_generation;
        for my $gen (@generation) {
            my ($base, $start_at) = @$gen;
            for my $i ($start_at..$#alphabet) {
                my $new_str = $base.$alphabet[$i];
                push @return, $new_str;
                push @next_generation, [$new_str, $i+1] if $i < $#alph
+abet;
            }
        }
        @generation = @next_generation;
    }
    @return;
}
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