But then how come this works without error? (assuming that the method getCells is declared in Brain)
What should happen if one of the classes has an AUTOLOAD? Also remember that you can add methods a class and remove them from it, at run time. You can even manipulate @ISA in all of your packages at runtime, completely changing your inheritance tree around at will. In fact, the inheritance tree is always manipulated on the fly, since there's no way to declaratively populate @ISA, even if that manipulation just so happens to commonly happen in implied BEGIN {} blocks.
It is impossible to make your scenario fail properly at compile time because Perl is simply too dynamic. Other similarly dynamic very high level languages have similar problems with such scenarios.
Makeshifts last the longest.
