Your &uplevel call don't work if base is compiled before you compile Sub::Uplevel, which it is, and you can't ever guarentee to load Sub::Uplevel before base from within your module, no matter how hard you try, which is annoying. Perl needs a real uplevel().
Anyhow, while this is a fun module, I fail to see what good it really does. I'd just stick with
use base 'Foo'; use Foo qw/ Import some stuff /;
use base 'Bar'; use Bar;
use base 'Buz'; use Buz ();
It's a few extra chars, but I and everyone else will know exactly what happens. I guess this is one of those "just because you can doesn't mean you should" situations Perl constantly provides us with. :-)
See perltoc if you don't know which perldoc to read!