### 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;