SQL>
SQL> declare
2 l_clob clob := '12345';
3 begin
4 dbms_lob.write( l_clob, 2, 3, 'AB' );
5 dbms_output.put_line( l_clob );
6 dbms_lob.write( l_clob, 2, 9, 'CD' );
7 dbms_output.put_line( l_clob );
8 end;
9 /
12AB5
12AB5 CD
PL/SQL procedure successfully completed.
SQL>
SQL> --