We recently did an upgrade, and this is what bit us:
- Hashes really are random now. They should have been before, but we had some CGI scripts that were using hashes to generate drop-downs. The drop-downs would always come out in the same order before because for very small hashes, the order wasn't really random. They will be different each time now.
- CGI.pm changes a lot. I think we jumped something like 40 versions when we upgraded. Read the change log and look for gotchas. Tab index feature added to CGI.pm might be one gotcha.
- DBI changed some default connect options along the way somewhere. I think the one that burned us was ChopBlanks.
The last thing is just to note that you'll need to re-compile all your supporting C code too for the newest versions of modules. So if you use XML modules, you'll probably need to get the newest version of expat and recompile, etc.
Our upgrade went very smooth, with the exception of the issues above. If you need to install a lot of modules too, you might try out Order your autobundle by dependency