my @list = (
     a => [ 'b', 'c', 'e' ],
     b => [ 'd' ],
     c => [ 'b', 'd' ],
     f => [ 'a' ],
);
print join (',', map { "'$_'" } f2(@list)),"\n";

##</code><code>##

sub f {
%r=@_;map{$m{$_}={map{$m{$_}||={};$_=>1}@{$r{$_}}}}keys%r;
reverse sort{my$t=$m{$b}{$a}-$m{$a}{$b};warn"$a,$b:$t";$t}keys%m;
}

##</code><code>##

f,e:0 at ...
a,f:0 at ...
b,a:1 at ...
c,b:1 at ...
d,c:1 at ...