Jesse posted the following a few minutes ago:

I've just uploaded the first release candidate for Perl 5.12.0 to
PAUSE. Shortly, you'll find it at:
                                  
    http://search.cpan.org/dist/perl-5.12.0-RC1/

SHA1 sums for this release are:
                               
    d6b0357667582e515335ed89d86bcecb348a07c6  perl-5.12.0-RC1.tar.bz2
    d2616e5c193377901cc930290168d08f10b3bd1d  perl-5.12.0-RC1.tar.gz 

The differences from RC0 are relatively small, consisting primarily of
documentation updates and a few small regression fixes.

It is imperative that you test this release candidate with any software
written in Perl which you use or maintain. If no "showstopper" class
bugs are found in the next 7 days, we will release a virtually identical
tarball as Perl 5.12.0.

You can find a list of errata - known bugs or modules known to be
incompatible with 5.12.0 - at http://dev.perl.org/perl5/errata.html