my %tree = (
            child1 => {},
            child2 => {
                       grandchild1 => { brood3 => {} },
                       grandchild2 => { brood1 => {}, brood2 => {} },
                      },
           );

say for 'Grandchildren:',
    map {
        my $x = $_;   #   <--- remove me!
        "$_: " . (ref $tree{$_} ? join ', ' =>
                                  map keys %{ $tree{$x}{$_} },
                                      keys %{ $tree{$_}     }
                                : q())
    } keys %tree;