How to write and call PL/SQL function in SQL script using WITH clause?

Example# 1:

WITH
FUNCTION voter(age NUMBER) Return VARCHAR2
IS
BEGIN
If(age > 18) THEN
RETURN 'Eligible for voting.';
ELSE
RETURN 'Not eligible for voting.';
END if;
END;
--

SELECT voter(19) FROM dual;
--

Output:

Eligible for voting. 

Note:

In this case PL/SQL function object not saved or created. We must write the function where we want to use. (See above example# 1.)