See, undefined behaviour is just that.. if you rely on it, you are submitting to surprises. Of course, your stance is a legit one, and I do think that cases like yours have to be considered; old code should not be broken lightheartedly, even if it was written with dirty practices. But I do not believe that this argument can outweigh significant reasons that favour a change.

I'm not saying that substr should be changed, rather than the current behaviour just becoming defined and documented. Whether there are any strong arguments in favour of a change in this specific instance is yet to be seen.

I do however feel that reliance on poorly documented or defined behaviour or even on outright bugs is too common a bad habit and anything that will teach people to refrain from it can only be a good thing in the long term.

After all, noone is forcing you to upgrade - Perl4, 5.0005_003 etc all still run fine. Old codebase does not have to be fixed right this instant.

I realize this is somewhat of an ivory tower opinion and that things in the realworld don't quite always work the way I would like them to. However, unless you begin somewhere and sometime, that's never going to change. And in Germany they say (roughly), a terrifying end is better than neverending terror.

In the end, I guess, it comes down to trying to find the best compromise. Legacy code should not hinder progress - but of course progress at all cost is no better either. Anyway, I think I'll quit this longwinded, meandering tangential post here before it goes much further.