A call to action for CPAN authors
CPAN authors should look at the smoke tests for their modules to ensure that they're passing on Perl 5.18. The hash randomization change (and a few others) has bitten many a module that may currently be relying on undefined hash behavior. If you haven't checked your modules recently, you may be in for a surprise.
The problem is significant, especially when compounded by the fact that a single misbehaving module can block many more modules that may depend on it.
As an example (not to finger-point, but just to illustrate): Crypt::DES, Crypt::IDEA, Crypt::Blowfish, and Crypt::Twofish had a single line of XS code that was incompatible with Perl 5.18 (which happened to not be related to hash randomization). As a result, all modules that depended on any of these modules also would fail to install on Perl 5.18. This includes Authen::Passphrase, and Crypt::OpenPGP, as well as dozens of others.
For awhile during Perl 5.17 development Perl 5 Porters were keeping a list of modules they were aware of that had bugs made evident by the hash randomization change. But given that we've been hearing warnings about relying on undefined behavior for years, and in particular warnings about hash randomization for many months, at some point it stops being Perl 5 Porters responsibility to keep track of modules that have bugs.
In many cases, these bugs are in modules' test suites, and not the modules' primary code. But whatever the case, we need to be diligent in ferreting out such problems and resolving them. Having a good portion of CPAN break because of a number of modules relying on undefined behavior is one of those "egg on our face" moments that we need to wipe off and put behind us.
If you know one of your modules has begun failing on Perl 5.18, I encourage you to fix it. If you know of an unmaintained module that has begun failing, follow up with the author, or seek to take over maintenance. Report problems to the RT system. Contribute patches. Let's work together to get this problem resolved as quickly as possible.