Something you do not want to do: install 5.8 and, knowing that it's binary incompatible with 5.6, decide to install the autobundle snapshot from the latter. That's a bad idea, especially if like me your autobundle has circa 1.400 entries... Of course, many of those are things I just checked out, or used to use but have abandonned since, and definitely don't need.
It's been going for hours, sometimes slowing down my box, regularly failing a module here and there, sometimes installing the same module several times (usually different versions), and of course you have the problem of interactive Makefile.PLs, notably those that checked for dependencies themselves and then go on to fetch them in their own instance of CPAN.
Now that it's at letter G I'd rather not interrupt it, but at any rate if that's something you were planning on I recommend installing as things break, or at least heavily editing your autobundle.
In any case, 5.8 appears to be working really well, and that's great