I'm not an expert with arrays but try this :
_array = [1,2,3]
_result = 0
_i = 0
#L
_value = _array select _i
_result = _result + _value
_i = _i + 1
?_i < count _array : goto "L"
hint format ["DEBUG = %1",_result]
But there must be a much easier way...