my @key_array = 'a' .. 'z';
my @set_array = ( ['a', 'b'],  ['e','f'], ['f','g'] );

my %set = map {$_ => {$_ => 1}} @key_array;

for my $s (@set_array) {
    @{$set{$_}}{@$s} = (1) x @$s for @$s;
}

my $saw = '';

print map "$_\n",
      grep $saw =~ s/^(?!.*$_)/$_ /,
      sort {length $b <=> length $a || $a cmp $b}
      map join(',', sort keys %$_),
      values %set;