### In Shoe.pm
package Shoe;

use strict;
use warnings;

# Shoes with holes in their soles.
my $num_holey = 0;

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;

    $num_holey++;

    print "Sole on this shoe has been worn out.\n";
    Shoe->how_many_holey();
}

# Class methods.
sub how_many_holey {
    my $class = shift;
    print "Currently, there are $num_holey holey shoes ($class).\n";
}

sub incr_num_holey {
    my $class = shift;
    $num_holey++;
}

1;

##</code><code>##

### In Boot.pm
package Boot;
use base qw( Shoe );

use strict;
use warnings;

# Boots are like Shoes, but have a heel.
# (Heel stuff not shown, for brevity.)

sub new {
    my $class = shift;
    my $self  = $class->SUPER::new();
    # ...
    return $self;
}

# Instance method that has to deal with a class variable.
sub wear_out_sole {
    my $self = shift;

    Shoe->incr_num_holey();
    print "Sole on this BOOT has been worn out.\n";
    Shoe->how_many_holey();
}

1;

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Boot;

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

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

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