Well, the CPANPLUS makefile says it is a unix thing, kind of -- something particular to the default perl installed with this redhat distro. From the second node I linked:

### there's issues with redhat 9.0's stock perl -- they applied some
### custom patches on their 5.8.0 and it breaks use constant a => sub 
+{};
if( $^O eq 'linux' and -e '/etc/redhat-release' and 
    $] == '5.008' and (grep /Red Hat, Inc/, values %Config::Config)
) {
    print loc("
### IMPORTANT! ######################################################

You are using perl $] supplied by RedHat, who have applied custom
patches that break various perl modules, including this one.

You will have to migrate to a perl without these flaws. You could
do this for example by building a perl installation by hand. 
You can obtain the sources from www.cpan.org.

We're sorry for the inconvenience.

#####################################################################

    ") . $/;

    require 5.008001;
}
[download]

I guess I could comment that out of the makefile and see if I get something usable... but nah, I'm going to let this alone for now.