my $var = [
           {
             "D1" => 1,
             "D2" => 2
           },
           {
             "D2" => 5, 
             "D3" => 6 
           },
];

my %combined;
foreach my $subhash (@$var) {
   foreach my $key (keys %$subhash) {
      $combined{$key} += $subhash->{$key};
   }
}

my @sorted_keys = sort { $combined{$b} <=> $combined{$a} }
                  keys %combined;

foreach my $key (@sorted_keys) {
   print("$key: $combined{$key}\n");
}