SQL & PL/SQL

I have a unix script which is invoking a sql file/anonymous block.
The sql file read a flat file through UTL_FILE utility and insert/update those data to table.
While invoking SQL script I put

 'SET DEFINE OFF' 

to get rid of the *'&'* char in the string (input file).
Now I have to process multiple files on a single run, I am invoking the SQL script by passing arguments like :

Invoke :

start xx.sql FILE_NAME

Inside xx.sql accessing the arguments like :

declare
y varchar2(20);
begin
dbms_output.put_line('Y= '||y);
y:='&1';
dbms_output.put_line('Y= '||y);
end;
/

It only works if I put

 'SET DEFINE ON' 

In this case any *'&'* char being treated as read from standard input.
Creating procedure/function is out of scope.
Can anyone suggest ... what should be the approach and is there a way to manage it ?