-- Package pck
FUNCTION get_value
  ( pv_plsql_expression IN VARCHAR2
  ) RETURN VARCHAR2
IS
  lv_value VARCHAR2(1000) := NULL;
BEGIN
  IF pv_plsql_expression IS NOT NULL
  THEN
    EXECUTE IMMEDIATE 'BEGIN :ret := ' || pv_plsql_expression || '; END;'
      USING OUT lv_value;
  END IF;
  RETURN lv_value;
END get_value;

SELECT pck.get_value('pck1.constant_name') FROM table;

SELECT pck1.constant_name FROM table;