The relevant section of CPAN.pm (at least in 5.8.4):

        $system = "$CPAN::Config->{tar} xvf $file";
        $CPAN::Frontend->myprint(qq{Using Tar:$system:\n});
        if (system($system)==0) {
            $CPAN::Frontend->myprint(qq{Untarred $file successfully\n}
+);
        } else {
            $CPAN::Frontend->mydie(qq{Couldn\'t untar $file\n});
        }
[download]

That system call is failing for some reason. I'd check to make sure that (a) the same tar command does not fail on the command line (try something like

/bin/tar xvf /home/ro...es/MP3-Tag-0.94.tar && echo ok || echo failed
[download]

), and (b) that there is no difference in content between the stuff untarred by your CPAN CGI script and the stuff untarred from the command line (use the directory form of diff). That may give you some clues for why the system call is failing. You could write a simple CGI script that just untars something via a similar system call, and check for its return value, and thereby begin to narrow down the problem (i.e. CPAN.pm vs. tar), but I admit that I'm grasping at straws here.