$rh_dtree = {
             'L1' => {
                       'L2b' => {
                                  'L2b1' => {},
                                  'L2b2' => {}
                                },
                       'L2a' => {
                                  L3b' => {
                                             L4a' => {}
                                          },
                                  L3a' => {}
                                }
                     },
           'L3b' => {
                      'L4a' => {}
                    },
           'L2b' => {
                      'L2b1' => {},
                      'L2b2' => {}
                    },
           'L2a' => {
                      'L3b' => {
                                 'L4a' => {}
                               },
                      'L3a' => {}
                    }
        };

##</code><code>##


Main();

Main
{
   my %dtree = init();
   processDtree(\%dtree);
}

sub processDtree
{
   my $rh_dtree = shift;
   
   foreach my $parent ( keys %{$rh_dtree} )
   {
      my @nodenames = ();
      hash_walk( \%{$rh_dtree->{$parent}},[],\@nodenames);
      print join("\n",@nodenames) . "\n";
exit;
   }
}

sub hash_walk
{
    my ($rh_dtree,$ra_keys,$ra_nodenames) = @_;
    
    while (my ($k, $v) = each %$rh_dtree) 
    {
       # keep keys ordered.
       push(@$ra_keys,$k);
       if ( scalar keys %{$v} ) 
       {
          hash_walk($v,$ra_keys,$ra_nodenames);
       }
    }
    push(@$ra_nodenames, join(".",@$ra_keys));
    @$ra_keys = ();
    
}

##</code><code>##

L2b.L2b1
L2b.L2b1.L2b2
L2b.L2b1.L2b2
L2b.L2b1.L2b2.L2a.L3b.L4a
L2b.L2b1.L2b2.L2a.L3b.L4a
L2b.L2b1.L2b2.L2a.L3b.L4a.L3a
L2b.L2b1.L2b2.L2a.L3b.L4a.L3a


##</code><code>##

nested-L2b
nested-L2b.nested-L2b1
nested-L2b.nested-L2b2
nested-L2a
nested-L2a.nested-L3b
nested-L2a.nested-L3b.nested-L4a
nested-L2a.nested-L3a