When "optional' is used with a struct member, C++ server seems to not return it correctly

namespace cpp thrifttest
namespace perl thrifttest

struct Pair {
    1: optional string a, 
    2: optional string b
}

struct Result {
    1: optional list<string> strList,
    2: optional list<Pair> pairList
}

service Test {

    Result listTest()

}

#include "gen-cpp/Test.h"
#include <thrift/protocol/TBinaryProtocol.h>
#include <thrift/server/TSimpleServer.h>
#include <thrift/transport/TServerSocket.h>
#include <thrift/transport/TBufferTransports.h>

#include <iostream>

using namespace std;

using namespace ::apache::thrift;
using namespace ::apache::thrift::protocol;
using namespace ::apache::thrift::transport;
using namespace ::apache::thrift::server;

using boost::shared_ptr;

using namespace  ::thrifttest;

class TestHandler : virtual public TestIf {
    public:
        TestHandler() {
            // Your initialization goes here
        }

        void listTest(Result& _return) {

            _return.strList.push_back("Test");
            _return.strList.push_back("one level list");
            cout << "strList size: " << _return.strList.size() << endl;

            Pair pair;
            pair.a = "Test";
            pair.b = "two level list";
            _return.pairList.push_back(pair);
            cout << "pairList size: " << _return.pairList.size() << endl;

            printf("call listTest\n");
        }

};

int main(int argc, char **argv) {
    int port = 9595;
    shared_ptr<TestHandler> handler(new TestHandler());
    shared_ptr<TProcessor> processor(new TestProcessor(handler));
    shared_ptr<TServerTransport> serverTransport(new TServerSocket(port));
    shared_ptr<TTransportFactory> transportFactory(new TBufferedTransportFactory());
    shared_ptr<TProtocolFactory> protocolFactory(new TBinaryProtocolFactory());

    TSimpleServer server(processor, serverTransport, transportFactory, protocolFactory);
    server.serve();
    return 0;
}

#!/usr/bin/perl

use v5.12;
use warnings;
use autodie;

use utf8;
use Data::Dumper;

use lib 'gen-perl';

use thrifttest::Test;
use thrifttest::Constants;
use thrifttest::Types;

use Thrift;
use Thrift::BinaryProtocol;
use Thrift::Socket;
use Thrift::BufferedTransport;


my $socket    = new Thrift::Socket('localhost', 9595);
my $transport = new Thrift::BufferedTransport($socket, 1024, 1024);
my $protocol  = new Thrift::BinaryProtocol($transport);
my $client    = new thrifttest::TestClient($protocol);


eval {
    $transport->open();
    my $result = $client->listTest;
    say Dumper($result);
    $transport->close();
};
say $@ if $@;

strList size: 2
pairList size: 1
call listTest

$VAR1 = bless( {
                 'pairList' => undef,
                 'strList' => undef
               }, 'thrifttest::Result' );