MessagePack Perl to C++ deserialisation

I'm new to messagepack, and I'm trying to take a hash in perl, serialize it using messagepack, write it to a file, pass that to c++ code where the file is read and deserialized into a map.

My perl code to generate the file is (note - I added an extra part to check I could read the file back in and deserialize it correctly in perl, although I don't really need to do this):

#! perl

use strict;
use warnings;

use Data::MessagePack;

my %hTestHash = ('AAAAAA' => '20020101',
                 'BBBBBB' => '20030907');

my $packed = Data::MessagePack->pack(\%hTestHash);

open my $fh, '>', 'splodge.bin' or die "Failed to open slodge.bin for write: $!";
print $fh $packed;
close $fh;

open my $fh2, '<', 'splodge.bin' or die "Failed to open slodge.bin for read: $!";
local $/;
my $file = <$fh2>;

my $hrTest = Data::MessagePack->unpack($file);

My c++ code to deserialize is then:

#include "msgpack.hpp"
#include <string>
#include <iostream>
#include <sstream>
#include <fstream>

int main(void)
{
  // Deserialize the serialized data.
  std::ifstream ifs("splodge.bin", std::ifstream::in);
  std::stringstream buffer;
  buffer << ifs.rdbuf();
  msgpack::unpacked upd;
  msgpack::unpack(&upd, buffer.str().data(), buffer.str().size());
  msgpack::object obj = upd.get();
  std::map<std::string, std::string> output_map;
  msgpack::convert(output_map, obj);

  string date = output_map.at("AAAAAA");

  return 0;
}

This produces a 2 element map in output_map, but it only contains garbage values - my program crashes out on output_map.at() with

{"▒▒▒▒▒▒"=>"▒▒▒▒▒▒▒▒", "▒▒▒▒▒▒"=>"▒▒▒▒▒▒▒▒"}
terminate called after throwing an instance of 'std::out_of_range'
  what():  map::at
Aborted

I've been unable to find any examples of this particular use case, and struggling to see what's wrong - is this a problem at the serialization end or (seems more likely) at the deserialization end?

EDIT: Thanks to @SinanÜnür for pointing out my mistake, which I have now updated in the question. This doesn't change the fact that the hash is being populated with garbage values, so the same exception is being thrown regardless of the key being searched for.

Finally managed to get it working with a combination of proper binary file reading and some munging with weird in-house data types that we have.

The functioning code (which deserializes the structure into a Map<string, msgpack::object>, requiring an extra step of deserializing each value when it is needed) is:

#include "msgpack.hpp"
#include "map.h"
#include <string>
#include <iostream>
#include <fstream>

void unpack_map(const msgpack::object& o, Map<string,msgpack::object>& results){
  ASSERT(o.type == msgpack::type::MAP);
  for (msgpack::object_kv *p = o.via.map.ptr, *pend = (o.via.map.ptr + o.via.map.size); p != pend; ++p)
    results[p->key.as<string>()] = p->val;
}

int main(void)
{
  streampos size;
  char * memblock;

  ifstream file ("splodge.bin", ios::in|ios::binary|ios::ate);
  if (file.is_open())
    {
      size = file.tellg();
      memblock = new char [size];
      file.seekg (0, ios::beg);
      file.read (memblock, size);
      file.close();

      cout << "the entire file content is in memory";
    }
  else cout << "Unable to open file";

  msgpack::unpacked upd;
  msgpack::unpack(&upd, memblock, size);
  msgpack::object obj = upd.get();

  if(obj.type != msgpack::type::MAP) {
    cout << ":(" << endl;
  } else {
    cout << ":)" << endl;
  }
  cout << "size: " << obj.via.map.size << endl;

  Map<string, msgpack::object> objectMap;
  unpack_map(obj, objectMap);

  msgpack::object val  = objectMap["BBBBBB"];
  string tmp = string(val.via.raw.ptr, val.via.raw.size);

  delete[] memblock;
  return 0;
}

where tmp takes the value 20030907