### 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;