You could do a count(*) instead

declare
  l_cnt integer;
begin
  select count(*)
    into l_cnt
    from items
   where id_referencia = :P2_REFERENCIA;

  if( l_cnt >= 1 )
  then
    return true;
  else 
    return false;
  end if;
end;

If you really wanted to do an exists, you'd do something like this. I don't know why you'd want to do this assuming that id_referencia is the primary key of the table. But you could

declare
  l_exists integer;
begin
  begin
    select 1
      into l_exists
      from dual
     where exists( select 1
                     from items
                    where id_referencia = :P2_REFERENCIA );
  exception
    when no_data_found
    then
      l_exists := 0;
  end;

  if( l_exists = 1 )
  then 
    return true;
  else
    return false;
  end if;
end;