Magento 2 Rest Api - Associative Array In Request Param

I am trying to create custom Api in magneto2. Its a POST CALL, in which i am trying to send associative array like below,

{
    "data": {       
        "testData": {
            "title": "Test 01",         
            "place": {
                "key": "value"
            }
        }
    }
}

For the above format i am giving the param annotation as string[]. Its working If i didn't pass place data in it. But if i am passing place data its throwing error like below,

Array to string conversion

Actual problem is i am not able to pass associative array in request param. Can anyone please help me on this?

I tried by creating an custom object type also. In that too i can give string[] type only. So i cannot add more and more json data in it.