Python

Alright, here are my solutions :)

1. Import Easter egg

import __hello__

Not the most technically interesting, but a fun Easter egg!

2. Class decorators

@print
@lambda _: "Hello World"
class Foo: 
    ...

Decorators are another way of calling a function, so can be abused to solve this task. You need to decorate a class rather than a function though, since a function definition requires parentheses!

3. Dunder methods

class Printer:
    __class_getitem__ = print

Printer["Hello World"]

There might be some other Dunder methods you can use to do this, although it's sort of difficult since most (e.g. __add__) only describe behaviour on the instance of the class.

4. More dunder methods

from _sitebuiltins import Quitter
Quitter.__add__ = print
exit + "Hello World"

Writing number 3 made me realise I just needed to find a class that already had an instance and change that. I'm sure there are many other cases of this, but here's one!