# the nested array refs
my $tree = [
           'Root', [
                 'Child1', [
                       'GrandChild1',
                       'GrandChild2'
                 ],
                 'Child2'
           ]
];

# the nested hash references
my $tree = {
           'Root' => {
                 'Child1' => {
                       'GrandChild1' => {},
                       'GrandChild2' => {}
                 },
                 'Child2' => {}
           }
};

##</code><code>##

# using arrays where 
# the array index is 
# the id refered to by 
# the parent_id
my $tree = [
        # parent_id, node
       [ undef, 'Root' ],
       [  0, 'Child1' ],
       [  0, 'Child2' ],
       [  1, 'GrandChild1' ],
       [  1, 'GrandChild2' ]
];

# using hashes in a similar manner
my $tree = [
       { id => 0, parent_id => undef, node => 'Root' },
       { id => 1, parent_id => 0, node => 'Child1' },
       { id => 2, parent_id => 0, node => 'Child2' },
       { id => 3, parent_id => 1, node => 'GrandChild1' },
       { id => 4, parent_id => 1, node => 'GrandChild2' }
];