Oracle – Printing the value of a variable in SQL Developer

I wanted to print the value of a particular variable which is inside an anonymous block. I am using Oracle SQL Developer. I tried using dbms_output.put_line. But it is not working. The code which I am using is shown below.

SET SERVEROUTPUT ON

DECLARE

  CTABLE USER_OBJECTS.OBJECT_NAME%TYPE;
  CCOLUMN ALL_TAB_COLS.COLUMN_NAME%TYPE;
  V_ALL_COLS VARCHAR2(500);

  CURSOR CURSOR_TABLE
    IS
    SELECT OBJECT_NAME 
    FROM USER_OBJECTS 
    WHERE OBJECT_TYPE='TABLE'
    AND OBJECT_NAME LIKE 'tb_prm_%';

  CURSOR CURSOR_COLUMNS (V_TABLE_NAME IN VARCHAR2)
    IS
    SELECT COLUMN_NAME
    FROM ALL_TAB_COLS
    WHERE TABLE_NAME = V_TABLE_NAME;

BEGIN

  OPEN CURSOR_TABLE;

  LOOP
    FETCH CURSOR_TABLE INTO CTABLE;
    EXIT WHEN CURSOR_TABLE%NOTFOUND;

    OPEN CURSOR_COLUMNS (CTABLE);

    V_ALL_COLS := NULL;

    LOOP
      FETCH CURSOR_COLUMNS INTO CCOLUMN;
      V_ALL_COLS := V_ALL_COLS || CCOLUMN;
      IF CURSOR_COLUMNS%FOUND THEN
        V_ALL_COLS := V_ALL_COLS || ', ';
      ELSE
        EXIT;
      END IF;
    END LOOP;

    DBMS_OUTPUT.PUT_LINE(V_ALL_COLS);

  END LOOP;
  CLOSE CURSOR_TABLE;

END;

And I am getting the output only as anonymous block completed.