sql_mode="oracle" cannot declare variable as constant

CREATE or replace  PROCEDURE FF_POSITION_BACK_PROC

(

  G_RET_CODE VARCHAR2

) AS

  G_PGMID         VARCHAR2(30) := 'FF_POSITION_BACK_PROC';

  G_PROC                  VARCHAR2(250);

  G_ZERO          CONSTANT NUMBER := 0;          

BEGIN

null;

END FF_POSITION_BACK_PROC;