Ok, in your case, because the Cow class has its own speak() method, this line:

Animal::speak('Cow');
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...is actually not being called, the one in Cow is instead.

Now, if you comment out or otherwise remove the Cow->speak() method from the Cow class, what will happen is, when you call Cow->speak(), that method will not be found, and since you are inheriting from Animal, Animal->speak() will be looked up instead, and if found, it'll call it, sending in the Cow class as the parameter to the inherited Animal->speak(). Observe:

use warnings;
use strict;

package Animal; {
    sub speak {
        my ($class) = @_;
        if ($class eq 'Cow'){
            print "$class says moooo!\n";
        }
    }
    sub run {
        my ($class) = @_;
        print "$class is running away!\n";
    }
}

package Cow; {
    our @ISA = qw(Animal);
}

package Rooster; {
    our @ISA = qw(Animal);
}

package main;

Cow->speak;
Rooster->run;
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Output:

Cow says moooo!
Rooster is running away!
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