...it probably shouldn't be this one. Instead, go read Breno G. de Oliveira's piece on hash randomization.
There are lots of important changes in 5.17 right now, and there may be some more between now and 5.18.0, which is due in Spring 2013. The one that you need to care about right now, though, is the change hash randomization. The changes to hash randomization are still themselves changing, but the bottom line is this: The order of things coming out of hashes is going to seem more random.
It's a pretty common mistake to believe that hash ordering is in some way
deterministic. The best guarantee you get from
perl is that if you call
values on a hash, the results correlate with one another. Relying
on any more ordering than that is a bug.
The changes in 5.18.0 will expose these bugs where they had been hidden before.
So, rather than restate the things that Breno said so well, I will once again tell you: Go read Breno's post.
If you want to be really helpful, you could look at the list of CPAN distributions affected by this change and help make sure they are patched before 5.18.0 arrives. Heck, maybe your own code is in there.