I am going through book "Intermediate Perl" and started with Object Oriented section. I have created a distribution Animal and inside it:

bin/pasture

use Animal;
use Cow;
use Horse;
use Sheep;

Cow->speak;
Horse->speak;
Sheep->speak;
[download]

lib/Animal.pm

package Animal;

use 5.006;
use strict;
use warnings;

sub speak
{
    my $class = shift;
    print "a $class goes ", $class->sound, "!\n";
}

sub sound
{
    die "'You have to define sound() in a subclass' found";
}
[download]

lib/Cow.pm

package Cow;

use 5.006;
use strict;
use warnings;
use Animal;
our @ISA = qw(Animal);

sub sound { "moooo" }
[download]

lib/Sheep.pm

package Sheep;

use 5.006;
use strict;
use warnings;
use Animal;
our @ISA = qw(Animal);

sub sound { "baaaah" }
[download]

lib/Horse.pm

package Horse;

use 5.006;
use strict;
use warnings;
use Animal;
our @ISA = qw(Animal);

sub sound { "neigh" }
[download]

When I run ~/localperl/bin/perl -Ilib bin/pasture I get following output:

a Cow goes moooo!
a Horse goes neigh!
a Sheep goes baaaah!
[download]

As per book, explanation of this behavior is:

What happens when we invoke Cow−>speak now?

First, Perl constructs the argument list. Here, it’s just Cow. Then Perl looks for Cow::speak. That’s not there, so Perl checks for the inheritance array @Cow::ISA. It finds @Cow::ISA contains the single name Animal.

Perl next looks for speak inside Animal instead, as in Animal::speak. That found, Perl invokes that method with the already frozen argument list, as if we had said:

Animal::speak('Cow'); #This is the part I am having trouble understanding. How did "Cow" become an argument when I didn't specify it anywhere?

As I mentioned in comment, I am unable to understand: How did "Cow" become an argument when I didn't specify it anywhere?

Can someone please help me understand this behavior? (Forgive me for the long post.)

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