use XML::DOM;

my $parser = new XML::DOM::Parser;
	my $root_node = $parser->parsefile($field_definition_file);

	##actually returns only one node
	my $simpleType_nodes=$root_node->getElementsByTagName("simpleType");
	$simpleType_nodes->item(0)->createElement("length", value=>$maxlength_hash{$field_name} );