A little digging with http://search.cpan.org, and perusal of libwww-perl as well AnyDBM.File.pm turned up no recognisable answers.

On to a Google search with the text "robot/rules-dbm.....FAILED test 8", which turned up this question and this answer, both from 1997.   Given the vintage and nature of the answer, it would be surprising if that helps you 8^(

Hmmm... Unless you plan " to implement persistent diskcaching of robots.txt and host visit information", I'm guessing you could ignore that one test failure, and continue on with make install.   Especially since the other 12 tests passed.

Update:
D'oh!   You said CPAN.pm, so do what good monk larsen says above.   Or install the module manually like so:

download and extract gzipped tarball
perl Makefile.pl
make
make test (see the same error, eh?)
make install
[download]

    cheers,
    Don
    striving toward Perl Adept
    (it's pronounced "why-bick")