The child class has a package variable called @ISA which tells it the parent class. This is an array because multiple inheritance is supported.

package Child {
   require Parent;
   our @ISA = "Parent";
}
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For some reason, setting @ISA directly like that is not very fashionable, and a lot of people prefer to use a module like base, parent, or superclass to do so. Using one of those modules ensures that @ISA gets set at compile-time rather than run-time, though in practice this feature of them is almost never important.