my $str = "One:Two:Three:four, One:Two:3:4, One:2:3:4, One:2:three";

##</code><code>##

$keywords = {
    One => {
        Two => { Three => four, 3 => 4 },
        2 => { 3 => 4, three => "" }
    }
};

##</code><code>##

my $n = 0;
sub print_keywords
{
    my $k = shift;
    for my $key (keys %$k) {
        print "    " x $n;
        print "$key\n";
        $n++;
        if (keys %{$k->{$key}}) {
            print_keywords($k->{$key});
        } elsif ($k->{$key}) {
            print "    " x $n, $k->{$key}, "\n";
        }
        $n--;
    }
}