If I understand you correctly, then I believe there are two problems with this,

1. This requires that the coder using my object knows about the internal details of my object, and that the coder always knows at run time exactly what kind of object they have. I want them to be able to do_something with any subclass of Foo without having to know which subclass it is.
2. I couldn't get your to work for me to work in all cases. When I modify the definition of $sub in the main package to call $b->do_something('bar'), I get the exact same error:

Not foo
Can't locate object method "do_something" via package "Foo"
 (perhaps you forgot to load "Foo"?) at ./foobar.pl line 20.