I think this does it -- test it fully, did it rather quick.

create or replace
procedure lob_replace( p_lob in out clob,
                       p_what in varchar2,
                       p_with in varchar2 )
as
    n        number;
    l_offset number := 1;
begin
loop
    n := dbms_lob.instr( p_lob, p_what, l_offset );
    if ( nvl(n,0) > 0 )
    then
        if ( (n+length(p_what)) < dbms_lob.getlength(p_lob) )
        then
           dbms_lob.copy( p_lob,
                          p_lob,
                          dbms_lob.getlength(p_lob),
                          n+length(p_with),
                          n+length(p_what) );
        end if;
                                                                                   
        dbms_lob.write( p_lob, length(p_with), n, p_with );
        if ( length(p_what) > length(p_with) )
        then
            dbms_lob.trim( p_lob,
               dbms_lob.getlength(p_lob)-(length(p_what)-length(p_with)) );
        end if;
        l_offset := l_offset + length(p_with);
    else
        exit;
    end if;
end loop;
end;
/
                                                                                   



ops$tkyte@ORA9IR2> insert into t values ( 'a b a c d e a' );
 
1 row created.
 
ops$tkyte@ORA9IR2> declare
  2      l_clob clob;
  3  begin
  4          select x into l_clob from t for update;
  5      lob_replace( l_clob, 'a', 1 );
  6  end;
  7  /
 
PL/SQL procedure successfully completed.
 
ops$tkyte@ORA9IR2> select * from t;
 
X
--------------------------------------------------------------------------------
1 b 1 c d e 1
 
ops$tkyte@ORA9IR2> update t set x = replace( x, 1, 'aaa' );
 
1 row updated.
 
ops$tkyte@ORA9IR2> select * from t;
 
X
--------------------------------------------------------------------------------
aaa b aaa c d e aaa
 
ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2> delete from t;
 
1 row deleted.
 
ops$tkyte@ORA9IR2> insert into t values ( 'e a b a c d e a e' );
 
1 row created.
 
ops$tkyte@ORA9IR2> declare
  2      l_clob clob;
  3  begin
  4          select x into l_clob from t for update;
  5      lob_replace( l_clob, 'a', 1 );
  6  end;
  7  /
 
PL/SQL procedure successfully completed.
 
ops$tkyte@ORA9IR2> select * from t;
 
X
--------------------------------------------------------------------------------
e 1 b 1 c d e 1 e
 
ops$tkyte@ORA9IR2> update t set x = replace( x, 1, 'aaa' );
 
1 row updated.
 
ops$tkyte@ORA9IR2> select * from t;
 
X
--------------------------------------------------------------------------------
e aaa b aaa c d e aaa e
 
 

replace, lobs were made "friendlier" since 8i came out

ops$tkyte%ORA10GR2> create table t ( x clob );

Table created.

ops$tkyte%ORA10GR2> declare
  2          l_clob clob;
  3  begin
  4          insert into t values ( 'hello world' ) returning x into l_clob;
  5          for i in 1 .. 10
  6          loop
  7                  dbms_lob.writeAppend( l_clob, 32000, rpad( '*', 32000, '*' ) );
  8          end loop;
  9          dbms_lob.writeAppend( l_clob, 5, 'end@@' );
 10          commit;
 11  end;
 12  /

PL/SQL procedure successfully completed.

ops$tkyte%ORA10GR2> select dbms_lob.getlength(x), substr( x, dbms_lob.getlength(x)-10 ) from t;

DBMS_LOB.GETLENGTH(X)
---------------------
SUBSTR(X,DBMS_LOB.GETLENGTH(X)-10)
-------------------------------------------------------------------------------
               320016
******end@@


ops$tkyte%ORA10GR2> update t set x = replace( x, 'end', 'ENDING' );

1 row updated.

ops$tkyte%ORA10GR2> select dbms_lob.getlength(x), substr( x, dbms_lob.getlength(x)-10 ) from t;

DBMS_LOB.GETLENGTH(X)
---------------------
SUBSTR(X,DBMS_LOB.GETLENGTH(X)-10)
-------------------------------------------------------------------------------
               320019
***ENDING@@
 

when others then
if (vDebug) then
err_num := SQLCODE;
err_msg := SUBSTR(SQLERRM, 1, 100);
insert into PEMS_PROD_2.SWN_POST_LOG (log_pk, create_date, REQUEST, notification_id) values(pems_prod_2.swn_post_log_seq.nextval,sysdate,
'err_num - '||to_char(err_num)|| ' error_msg - '|| err_msg, v_notification_id);
commit;
else 
null;
end if;
end;

begin
   log_error_every_time_debug_be_darned_using_autonomous_transaction();
   RALSE;
end;

ops$tkyte%ORA11GR2> create table t
  2  ( x int constraint check_x check (x>0),
  3    y int constraint check_y check (y>0)
  4  );

Table created.

ops$tkyte%ORA11GR2> 
ops$tkyte%ORA11GR2> create or replace procedure p( p_x in int, p_y in int )
  2  as
  3  begin
  4      insert into t values ( p_x, p_y );
  5      begin
  6          update t set y = -p_y where x = p_x;
  7      exception
  8          when others then null;
  9      end;
 10  end;
 11  /

Procedure created.

ops$tkyte%ORA11GR2> exec dbms_trace.set_plsql_trace( dbms_trace.trace_all_exceptions );

PL/SQL procedure successfully completed.

ops$tkyte%ORA11GR2> exec p(1,1)

PL/SQL procedure successfully completed.

ops$tkyte%ORA11GR2> exec p(-1,-1)
BEGIN p(-1,-1); END;

*
ERROR at line 1:
ORA-02290: check constraint (OPS$TKYTE.CHECK_Y) violated
ORA-06512: at "OPS$TKYTE.P", line 4
ORA-06512: at line 1

<b>Notice how the first execution appears to have succeeded - but we know it did not, the exception was swallowed</b>

ops$tkyte%ORA11GR2> select callstack
  2    from sys.plsql_trace_events
  3   where event_kind = 52
  4   order by runid, event_seq
  5  /

CALLSTACK
-------------------------------------------------------------------------------
----- PL/SQL Call Stack -----
  object      line  object
  handle    number  name
0x3a76fe2c         6  procedure OPS$TKYTE.P
0x376a09cc         1  anonymous block

----- PL/SQL Call Stack -----
  object      line  object
  handle    number  name
0x3a76fe2c         4  procedure OPS$TKYTE.P
0x374d0e80         1  anonymous block

SQL> create or replace
  2  procedure lob_replace( p_lob in out clob,
  3                         p_what in varchar2,
  4                         p_with in varchar2 )
  5  as
  6      n    number;
  7  begin
  8      n := dbms_lob.instr( p_lob, p_what );
  9      if ( nvl(n,0) > 0 )
 10      then
 11          dbms_lob.copy( p_lob,
 12                         p_lob,
 13                         dbms_lob.getlength(p_lob),
 14                         n+length(p_with),
 15                         n+length(p_what) );
 16
 17          dbms_lob.write( p_lob, length(p_with), n, p_with );
 18          if ( length(p_what) > length(p_with) )
 19          then
 20              dbms_lob.trim( p_lob,
 21                 dbms_lob.getlength(p_lob)-(length(p_what)-length(p_with)) );
 22          end if;
 23      end if;
 24  end;
 25  /

Procedure created.

SQL>
SQL>
SQL> drop table t purge;

Table dropped.

SQL> create table t ( c clob );

Table created.

SQL>
SQL> insert into t values ( 'a b a c d e a' );

1 row created.

SQL>
SQL> declare
  2   l_clob clob;
  3   begin
  4    select c x into l_clob from t for update;
  5    lob_replace( l_clob, 'a', 1 );
  6    end;
  7    /

PL/SQL procedure successfully completed.

SQL>
SQL> select * from t;

C
--------------------------------------------------------------------------------
1 b a c d e a

1 row selected.

SQL>