Adding const modifier to a variable declaration will protect it from being accidentally modified later in the code. CTL validator will report an error on any attempt to assign a value to a constant. Note that modifications via function calls, e.g. clear(), are not checked.

const integer INT_CONSTANT = 10;
const string MY_ID = "ABC";
const string[] LIST_CONSTANT = ["a", "b", "c"];

INT_CONSTANT = 11; // error
MY_ID = ""; // error
LIST_CONSTANT[0] = "x"; // error
clear(LIST_CONSTANT); // not checked