_at_ command does not work in SQL PLUS (OS HP-unix)

I have a sql file (abc.sql) which contains some code. I am able to run this using "run" command as SQL> run abc.sql
  1* select 1 from dual

         1

SQL> But not able to execute using "_at_" command. If i execute using _at_ it just return to the SQL prompt without executing this file.

SQL>  _at_abc.sql
SQL>
SQL>

Could you please help me in resolving this issue?

FYI, I m using Oracle 8.1.7.4.0 on HP-unix (Tru64 UNIX V5.1B (Rev. 2650) Content of abc.sql is

select 1 from dual
/

Thanks and Regards,
Chandra  

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