Did you try other serializers?

Apache::Session::Flex allows you to choose other serializers (such as Apache::Session::Serialize::YAML) that may perhaps be able to work with trees.

Update: Using ikegami's script but replacing Storable with YAML seems to work:

use strict;
use warnings;

use Tree         qw( );
use YAML         qw( Dump Load );
use Data::Dumper qw( Dumper );

my $tree = Tree->new( 'root' );
my $child = Tree->new( 'child' );
$tree->add_child( $child );

print(Dumper(Load(Dump($tree))));
[download]

Output:

$VAR1 = bless( {
                 '_parent' => \undef,
                 '_value' => 'root',
                 '_children' => [
                                  bless( {
                                           '_parent' => $VAR1,
                                           '_value' => 'child',
                                           '_children' => [],
                                           '_meta' => {},
                                           '_depth' => '1',
                                           '_error_handler' => sub { "
+DUMMY" },
                                           '_width' => '1',
                                           '_handlers' => {
                                                            'value' =>
+ [],
                                                            'add_child
+' => [],
                                                            'remove_ch
+ild' => []
                                                          },
                                           '_root' => $VAR1,
                                           '_height' => '1',
                                           '_last_error' => undef
                                         }, 'Tree' )
                                ],
                 '_meta' => {},
                 '_depth' => '0',
                 '_error_handler' => $VAR1->{'_children'}[0]{'_error_h
+andler'},
                 '_width' => '1',
                 '_handlers' => {
                                  'value' => [],
                                  'add_child' => [],
                                  'remove_child' => []
                                },
                 '_root' => $VAR1,
                 '_height' => '2',
                 '_last_error' => undef
               }, 'Tree' );
[download]