An example of using calldata parameter in internal functions.

calldata_example.sol

contract C {
    function f(uint[] calldata arr) external returns(uint) {
        return sum(arr);
    }
    function sum(uint[] calldata arr) internal returns(uint _sum) {
        for(uint i = 0; i < arr.length; i++) {
            _sum += arr[i];
        }
    }
}