malformed JSON string Error while passing JSON from AngularJS

I am trying to pass JSON string in ajax request. This is my code.

    NewOrder =  JSON.stringify (NewOrder);
    alert (NewOrder);

    var req = {
        url: '/cgi-bin/PlaceOrder.pl',
        method: 'POST',
        headers: { 'Content-Type': 'application/json'},
        data: "mydata="+ NewOrder
    };  

    $http(req)
    .success(function (data, status, headers, config) {
        alert ('success');
    })
    .error(function (data, status, headers, config) {
        alert (status);
        alert (data);
        alert ('Error')
    });

alert (NewOrder) gives -

{"ItemList":[{"ItemName":"Quality Plus Pure Besan 500 GM","Quantity":1,"MRP":"28.00","SellPrice":"25.00"}],"CustomerID":1,"DeliverySlot":2,"PaymentMode":1}

Which seems to be a valid JSON string.

But in script side I am getting following error. in this line

my $decdata = decode_json($cgi->param('mydata'));

malformed JSON string, neither array, object, number, string or atom, at character offset 0 (before "(end of string)")

Can please someone help me why i am getting this error?