SQL> select * from v$version
  2  /

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.1.0.4.0 - 64bi
PL/SQL Release 10.1.0.4.0 - Production

SQL> CREATE OR REPLACE TRIGGER change_hist
  2    AFTER CREATE ON APPS.SCHEMA
  3  DECLARE
  4  BEGIN
  5    if DICTIONARY_OBJ_TYPE in ('PROCEDURE', 'FUNCTION',
  6                               'PACKAGE', 'PACKAGE BODY', 'TYPE') then
  7  
  8       INSERT INTO SOURCE_HIST
  9              SELECT sysdate, user_source.* FROM USER_SOURCE
 10               WHERE  TYPE = DICTIONARY_OBJ_TYPE
 11                 AND  NAME = DICTIONARY_OBJ_NAME;
 12    end if;
 13  EXCEPTION
 14    WHEN OTHERS THEN
 15         raise_application_error(-20000, SQLERRM);
 16  END;
 17  /

Warning: Trigger created with compilation errors.

SQL> show err
Errors for TRIGGER CHANGE_HIST:

LINE/COL ERROR
-------- -----------------------------------------------------------------
6/6      PL/SQL: SQL Statement ignored
9/28     PL/SQL: ORA-00904: "DICTIONARY_OBJ_NAME": invalid identifier

SCOTT@orcl_11g> CREATE TABLE SOURCE_HIST		    -- Create history table
  2    AS SELECT SYSDATE CHANGE_DATE, USER_SOURCE.*
  3    FROM   USER_SOURCE WHERE 1=2;

Table created.

SCOTT@orcl_11g> 
SCOTT@orcl_11g> CREATE OR REPLACE TRIGGER change_hist	     -- Store code in hist table
  2    AFTER CREATE ON SCOTT.SCHEMA	     -- Change SCOTT to your schema name
  3  DECLARE
  4  BEGIN
  5    if DICTIONARY_OBJ_TYPE in ('PROCEDURE', 'FUNCTION',
  6  				  'PACKAGE', 'PACKAGE BODY', 'TYPE') then
  7  	  -- Store old code in SOURCE_HIST table
  8  	  INSERT INTO SOURCE_HIST
  9  		 SELECT sysdate, user_source.* FROM USER_SOURCE
 10  		  WHERE  TYPE = ORA_DICT_OBJ_TYPE
 11  		    AND  NAME = ORA_DICT_OBJ_NAME;
 12    end if;
 13  EXCEPTION
 14    WHEN OTHERS THEN
 15  	    raise_application_error(-20000, SQLERRM);
 16  END;
 17  /

Trigger created.

SCOTT@orcl_11g> show errors
No errors.
SCOTT@orcl_11g>