If you call My::Class->foo, then foo gets My::Class as its first argument, allowing it to know which class it was called as a method of. This isn't terribly important in most cases, but, without it, things like polymorphic constructors get a lot trickier, since they don't know whether they were called on the base class or a subclass.

If you call $some_obj->foo, then foo gets $some_obj as its first argument, which is extremely important if you want to be able to access the object's instance data.

If you call My::Class::foo, then it gets only the arguments you explicitly provide, the same as if you imported it and then called foo()¹. But you probably don't really want to do that. Writing subs that can be called in both OO and non-OO fashions adds complexity and, in most cases, provides little or no benefit.

¹ Note that I didn't prefix the function call with &. It's not needed in Perl 5 and has non-obvious side-effects which you almost certainly don't need. Only use the & prefix on function calls if you know exactly what the effects are and why you want those effects.