Don't throw warnings if perl-5.8.0 has been installed with Perlbrew.

I brewed perl-5.8.0 awhile back and had been getting warnings every time I used perlbrew:

```
$ perlbrew list
Argument "" isn't numeric in sprintf at /usr/local/share/perl/5.10.1/App/perlbrew.pm line 1114.
  perl-5.17.6
  perl-5.8.0
  perl-5.8.0@5.8.0
  perl-5.8.3
  perl-5.8.3@5.8.3
  perl-5.8.8
```

This patch should safely make the warning go away.