SQL & PL/SQL

I'd like to pass a procedure as a parameter to a sqlplus file and execute the procedure.

The sqlplus file, test.sql, looks like:

set serveroutput on
set verify off
set feedback off
set echo off

spool output.txt
begin
  dbms_output.put_line('Start~&1'||'~'||user);
  &1;
  dbms_output.put_line('END~&1');
end;
/
spool off

A simple procedure:

create or replace procedure proc1( p_arg in varchar2 default 'n' )
as
begin
  if nvl(p_arg,'n') = 'n' then
    dbms_output.put_line('Success');
  else
    dbms_output.put_line('Failed'); 
  end if;
end proc1;
/

In sqlplus:

SQL> @test.sql proc1  --This works
SQL> @test.sql proc('x')  --This doesn't work
SQL> @test.sql proc(''x'') --This also doesn't work

I'd appreciate any help or alternatives.