I was looking at the spamassin.spec file the other day in order to transfer
that build sequence over to the Gentoo Linux ebuild (to fix a couple bugs),
and I noticed that the build sequence specified in the spec file was not
working right.  I tracked down the root cause to the value of the SITEPREFIX
var that MakeMaker 6.05 is putting into the Makefile:

In earlier versions (such as 5.45) MakeMaker put the literal value $(PREFIX)
into this variable (unexpanded).  In MakeMaker 6.05, SITEPREFIX has the
expanded value of the PREFIX=$RPM_BUILD_ROOT/%{_prefix} setting stored
literally, so the "override" in the next command no longer has any function,
and the scripts get the wrong perl-lib dir in them.

I fixed the problem by forcing the value of SITEPREFIX to $(PREFIX) in
the "perl Makefile.PL" step.  I haven't tried this for building an RPM,
but it works great for my ebuild, so I figure a change like this is needed
in the spec file (I inserted the "SITEPREFIX=..." bit into this line):

  %{__perl} Makefile.PL INST_PREFIX=%{_prefix} SITEPREFIX=\$(PREFIX)
INST_SYSCONFDIR=%{_sysconfdir} PREFIX=$RPM_BUILD_ROOT/%{_prefix}
SYSCONFDIR=$RPM_BUILD_ROOT/%{_sysconfdir} < /dev/null

While you're at it, if the </dev/null bit is supposed to keep the the build
from prompting the user about the RAZOR checks, I'd suggest changing this to
add the "RUN_RAZOR1_TESTS=n RUN_RAZOR2_TESTS=n" settings instead.