ZMQ.REQ/REP problem(client)

my $context = ZeroMQ::Context->new();
my $json = JSON->new->allow_nonref(1);

my $socket = $context->socket(ZMQ_REQ);
$socket->connect($server_url);

for(my $i = 0;$i < 10;$i++) {
    $socket->send($i);
    my $answer = $json->decode($socket->recv()->data);
}