### In Shoe.pm
package Shoe;

use strict;
use warnings;

# Shoes with holes in their soles.
my %num_holey;

sub new {
    my $class = shift;
    my $self  = {};
    # ...
    bless $self, $class;
    return $self;
}

# Instance method that happens to access a class variable.
sub wear_out_sole {
    my $self = shift;

    $self->incr_num_holey() ; #$num_holey++;

    print "Sole on this " . ref($self) . " has been worn out.\n";
    $self->how_many_holey();
}

# Class methods.
sub how_many_holey {
    my $self = shift;
    local $_;
    print "Currently, there are $num_holey{$_} holey "
       . "$_(s).\n" for keys %num_holey;
}

sub incr_num_holey {
    my $self = shift;
    $num_holey{ref($self)}++;
}

1;

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use Shoe;
use Boot;

{package Shoe_Rack;
 sub new{
    my $class = shift;
    my $self  = {};
    # ...
    bless $self, $class;
    return $self;
 }
 sub Buy{
    my $self = shift;
    my $TypeToBuy = shift or die "What type to buy ?\n";

    $self->{COUNT}{$TypeToBuy}++; # Count whatever we buy
    my $newthing = $TypeToBuy->new();
    push @{$self->{COLLECTION}}, $newthing;
    return $newthing;
 }
 sub getcollection{
    my $self = shift;
    return  @{$self->{COLLECTION}};
 }
 1;
}


my $s = Shoe->new();
$s->wear_out_sole();

my $b = Boot->new();
$b->wear_out_sole();


my $b2 = Boot->new();
$b2->wear_out_sole();

my $Rack = new Shoe_Rack;
my $s1 =$Rack->Buy("Shoe");
$Rack->Buy("Shoe");
$Rack->Buy("Boot");
$Rack->Buy("Boot");
$Rack->Buy("Shoe");
$_->wear_out_sole() for $Rack->getcollection();