While your advice for when to write tests seems spot on—before writing (much) code, and when bugs are found—your advice for when not to write them seems suspect. Even when a complete, working application is in place, it's appropriate to buttress it with tests—they will help with maintenance, so that changes can be seen not to break existing behaviour, and, well, testing, perhaps revealing that an application that seemed to be both complete and working is neither. Since writing tests, especially in Perl, can be very cheap, and need not be done all at once, it seems harmless at worst.