Example code for MongoDB setting left and right values for modified preorder tree traversal. Problem and data are from here: http://www.sitepoint.com/hierarchical-data-database-2/

mptt.pl

#!/usr/bin/perl -w

=data
db.data.drop();
db.data.insert({_id: "Food"});
db.data.insert({_id: "Fruit", parent: "Food"});
db.data.insert({_id: "Red", parent: "Fruit"});
db.data.insert({_id: "Cherry", parent: "Red"});
db.data.insert({_id: "Yellow", parent: "Fruit"});
db.data.insert({_id: "Banana", parent: "Yellow"});
db.data.insert({_id: "Meat", parent: "Food"});
db.data.insert({_id: "Beef", parent: "Meat"});
db.data.insert({_id: "Pork", parent: "Meat"});

=cut

use strict; 
use warnings;

use MongoDB;

sub update_tree {
    my $conn = MongoDB::Connection->new(host => "localhost:27017");	
	my $database = $conn->get_database("test");
	my $collection = $database->get_collection("data");
	update_node($collection, "Food", 1);
}

sub update_node {
	my ($collection, $node_id, $left) = @_;
	my $right = $left + 1;
	my @cursor = $collection->find({parent => $node_id})->fields({_id => 1})->all();
	foreach my $next (@cursor) {
		$right = update_node($collection, $next->{_id}, $right) + 1;
	}
	$collection->update({_id => $node_id}, {'$set' => {left => $left, right => $right}});
	return $right;
}

update_tree();