|Problems? Is your data what you think it is?|
3) Always start your elsif or else block on a new line.
That seems to be a pure stylistic choice, cuddled else don't decrease robustness, efficiency or maintainability -- they just make some diffs a tiny bit larger.
9) Never use $_ or the invisible equivalent.
That's just stupid. If the scope for $_ is small, there's no reason to avoid it. Even more, if you don't want to use $_, you are doomed not to use map, grep and friends from List::Util. Which can make code much more maintainable.
I for one find the first one much more readable and maintainable, because it has less boilerplate code.
Unless and until could be very confusing.
They could be, if you used them in a stupid way. Like with a negated condition. But so could be the if and while equivalents. I agree about unless and else blocks though.
What I don't like about your list is that it forces people to write very unperlish code, and (implicitly) forbidding map and grep seems like you are afraid of their power. Maybe that's because you haven't understood them yet?
Anyway, here are my top 3:
1) Don't do stupid things
It's usually pretty obvious if code will be hard to maintain later on. Convoluted call graphs, 50-char regexes without whitespaces and excessive use of magic literals all indicate that you don't want to end up maintaining the code. So don't do that. But you don't need a rule for everything if you apply your common sense.
Which brings me to my second point:
2) Think about what you write
That one is pretty obvious, no?
3) Be consistent
In many ways it doesn't matter much how exactly your programming style looks like, as long as it's consistent. If you don't code alone, make sure to agree on some style with your peers.