I don't know how SWIG handles this, or even if it handles "output parameters" correctly, but as a first shot, make sure $response is already a string of size $respsize before calling the function, and check the output of sendMessage (looks like a status code).
my $response = " " x 100; # 100 spaces + 0 char
my $respsize = length $response; # 100
my $status = $c->sendMsg("hello",$response,$respsize);
print "status = $status, response is '$response'\n";