Show message right after command show in code

Question

I have some procedure in PL/SQL like:

set serveroutput on
DECLARE
  p_data          DATE;
  v_line          VARCHAR2(2000);
BEGIN
  v_line := 'START';
  dbms_output.put_line (v_line);

---------- 1 --------------------------
  WRT.TL_XTERN.process(
      p_data_d => p_data_d
  );
  v_line := ' WRT.TL_XTERN.process ended';
  dbms_output.put_line (v_line);

---------- 2 --------------------------   
  WRT.TL_MAP.process(
      p_data_d => p_data_d
  );
  v_line := ' WRT.TL_MAP.process ended';
  dbms_output.put_line (v_line);

  v_line := 'STOP';
  dbms_output.put_line (v_line);      
END;
/

Now, when I run it in sqlplus, all messages appear after both process ended. How can I get each message exactly when it appears in code.