for each item in items:
    for each keyword in item.keywords:
        frequency{keyword}++

for each item in items:
    sort keywords by frequency
    push hierarchy{sorted keywords}, item

##</code><code>##

  item = 1, keywords = a b c
  item = 2, keywords = b c
  item = 3, keywords = d b

##</code><code>##

a = 1, b = 3, c = 2, d = 1

##</code><code>##

b/c/2
b/c/a/1
b/d/3

##</code><code>##

my @items;

push @items, { name => 1, keywords => [ qw( b   ) ] },
             { name => 2, keywords => [ qw( b c ) ] },
             { name => 3, keywords => [ qw( b c ) ] },
             { name => 4, keywords => [ qw( b d ) ] },
             { name => 5, keywords => [ qw( e   ) ] },
             { name => 6, keywords => [ qw( e f ) ] };

my %freq;
for (@items) {
    for (@{ $_->{keywords} }) {
        $freq{$_}++;
    }
}

my @nodes;
for (@items) {
    my @keywords = sort { $freq{$b} <=> $freq{$a} }
                   @{ $_->{keywords} };
    my $path = join "/", @keywords, $_->{name};
    push @nodes, $path;
}

print "$_\n" for sort @nodes