This magical @ISA variable (pronounced "is a" not "ice-uh"), declares that Cow "is a" Animal. Note that it's an array, not a simple single value, because on rare occasions it makes sense to have more than one parent class searched for the missing methods. You'll learn more about that later.
 ISA is actually a linguistic term. Once again, Larry Wall's background as a linguist has come back to influence Perl.
If Animal also had an @ISA, you can check there too. Typically, each @ISA has only one element (multiple elements means multiple inheritance and multiple headaches), so you get a nice tree of inheritance. (There is also inheritance through UNIVERSAL and AUTOLOAD; see the perlobj manpage for the whole story.)
 The search is recursive, depth-first and left to right in each @ISA.
When you turn on use strict, you'll get complaints on @ISA because it's not a variable containing an explicit package name, nor is it a lexical (my) variable. You can't make it a lexical variable though: it has to belong to the package to be found by the inheritance mechanism.
There are a couple of straightforward ways to handle the declaration and setting of @ISA. The easiest is to just spell out the package name:
@Cow::ISA = qw(Animal);
You can also allow it as an implicitly named package variable:
package Cow; use vars qw(@ISA); @ISA = qw(Animal);
If you're on a recent-enough Perl (5.6 or later), you can use the our declaration to shorten it to:
package Cow; our @ISA = qw(Animal);
However, if you think your code might be used by people stuck with Perl 5.005 or earlier, it's be best to avoid our.
If you're bringing in the class from outside, via an object-oriented module, change:
package Cow; use Animal; use vars qw(@ISA); @ISA = qw(Animal);
package Cow; use base qw(Animal);
That's pretty darn compact. Furthermore, use base has the advantage that it's performed at compile time, eliminating a few potential errors from setting @ISA at runtime, like some of the other solutions.