watch out for cyclic structures.

use strict; use warnings; 
use Data::Dumper; 
no warnings 'once' ; 
  local $Data::Dumper::Sortkeys=1;
  local $Data::Dumper::Indent=1; 
use warnings; 
my $c={}; 
$c->{'1top'}={name=>'1top',childs=>[],parent=>undef}; 
makechild(name=>'child1',parent=>'1top'); 
makechild(name=>'child2',parent=>'1top'); 
makechild(name=>'child21',parent=>'child2'); 
makechild(name=>'child22',parent=>'child2'); 

print Dumper($c);


sub makechild{ 
  my %args=@_; 
  my $node={}; 
  $c->{$args{name}}=$node;
  $node->{name}=$args{name}; 
  $node->{childs}=[];
  my $parent=$c->{$args{parent}};  
  $node->{parent}=$parent;
  push @{$parent->{childs}},$node;
}
[download]

I think Dumper does it with a 'global' hash of all places already visited.

$VAR1 = {
  '1top' => {
    'childs' => [
      {
        'childs' => [],
        'name' => 'child1',
        'parent' => $VAR1->{'1top'}
      },
      {
        'childs' => [
          {
            'childs' => [],
            'name' => 'child21',
            'parent' => $VAR1->{'1top'}{'childs'}[1]
          },
          {
            'childs' => [],
            'name' => 'child22',
            'parent' => $VAR1->{'1top'}{'childs'}[1]
          }
        ],
        'name' => 'child2',
        'parent' => $VAR1->{'1top'}
      }
    ],
    'name' => '1top',
    'parent' => undef
  },
  'child1' => $VAR1->{'1top'}{'childs'}[0],
  'child2' => $VAR1->{'1top'}{'childs'}[1],
  'child21' => $VAR1->{'1top'}{'childs'}[1]{'childs'}[0],
  'child22' => $VAR1->{'1top'}{'childs'}[1]{'childs'}[1]
};
[download]

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