sub genIterator {
    my ($len,$count) = @_;
    my @list = ();
    return sub {
        if (@list == 0 || @list < $len) {
            push @list, 0;
        } else {
            $list[-1]++;
            while (@list && $list[-1] >= $count) {
                pop @list;
                $list[-1]++ if @list;
            }
        } 
        return @list ? join("", @list) : undef;
    }

}

##</code><code>##

my $iter = genIterator(4,3);
print while defined($_ = $iter->());