113.2.raku

my $tree = {
  v => 1,
  l => {
    v => 2,
    l => {
      v => 4,
      r => { v => 7 },
    },
    r => {
      v => 3,
      l => { v => 5 },
      r => { v => 6 },
    }
  },
};

sub walk($hash, &block)
{
  $hash.█
  .&walk(&block) for grep *.so, $hash<l>, $hash<r>;
}

my $sum = sum gather $tree.&walk({ take .<v> });
$tree.&walk({ .<v> = $sum - .<v> });

dd $tree;