# with dups
  my @inner_keys = map { keys %$_ } values %hash;

  # without dups
  my @uniq_inner_keys = do
  {
      my %inner_keys;
      while ( my ( undef, $href ) = each %hash )
      {
          @inner_keys{ keys %$href } = ();
      }
      keys %inner_keys;
  };