SQL & PL/SQL :: How To Use Default Value

I want to use default value in pl/sql , i'm executing below code

SET serveroutput ON
DECLARE
l_deptno INTEGER DEFAULT 10;
v_dname VARCHAR2(200);

[Code]...

SQL> @test.sql
Enter value for deptno: 40
dname value=OPERATIONS

I tried with NULL to use its default value but it uses NULL(which does make sense)

SQL> @test.sql
Enter value for deptno:
error

How to use the default value of the passed argument?

expected O/P
dname value=ACCOUNTING