send large string into clob column

I want to send a large string (80.000 bytes and more; base64 encoded image) from an html form or a simple link to a stored procedure that should insert the string into a clob column, but all I get is a "bad request" response from server. How can I achieve this? (I'm using Oracle 9.2; No Apex!)

Example:

http://example.com/pls/dad_test/mgintern.insert_string?p_string=aaaaaaaaaaaaaaaaaaaaaaaaaaaa... (80.000 "a"s)

Stored Procedure:

CREATE OR REPLACE PROCEDURE insert_string

  (

   P_string IN MGINTERN.TEST_TAB.IMAGE%TYPE

  ) IS

BEGIN

  INSERT INTO MGINTERN.TESTEMPFANG

    (

     IMAGE

    )

  VALUES

    (

     P_STRING

    );

  htp.p('image saved');