create table employees as select * from hr.employees

create or replace package pkg authid definer
is
   g_force_upper boolean := false;
end;

Without result cache

create or replace function last_name ( 
   p_employee_id in integer 
) return varchar2 is 
   l_name varchar2(1000); 
begin 
   select last_name 
     into l_name 
     from employees 
    where employee_id = p_employee_id; 
 
   return 
      case 
         when pkg.g_force_upper then 
            upper(l_name) 
         else l_name 
      end; 
end;

begin
   dbms_output.put_line (last_name (100));
end;

begin
   pkg.g_force_upper := true;
   dbms_output.put_line (last_name (100));
end;

begin
   pkg.g_force_upper := false;
end;

With result cache

create or replace function last_name ( 
   p_employee_id in integer, 
   p_force_upper_case in boolean default false 
) return varchar2 
   result_cache 
is 
   l_name varchar2(100); 
begin 
   select last_name 
     into l_name 
     from employees 
    where employee_id = p_employee_id; 
 
   return 
      case 
         when p_force_upper_case then 
            upper(l_name) 
         else l_name 
      end; 
end;

begin
   dbms_output.put_line (last_name (100));
end;

Dirty Data!

begin 
   pkg.g_force_upper := true; 
   dbms_output.put_line(last_name(100)); 
end;