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use strict;
use warnings;

...
$tree->add_child( $child );

print(Dumper(Load(Dump($tree))));

$VAR1 = bless( {
                 '_parent' => \undef,
                 '_value' => 'root',
...
                 '_height' => '2',
                 '_last_error' => undef
               }, 'Tree' );