create or replace trigger after_logon_if_sqlldr
  after logon on database
declare
  v_program  varchar2(64);
begin
  select program
  into   v_program
  from   v$session
  where  audsid = sys_context ('userenv', 'sessionid');
  if v_program = 'sqlldr.exe' then
    -- use one or the other of the below, not both
    execute immediate 'alter session set nls_numeric_characters = ''.,''';  
    -- or:
    execute immediate 'alter session set nls_numeric_characters = '',.''';  
  end if;
end after_logon_if_sqlldr;
/