$ perl -Mstrict -Mwarnings -le '
    package MyDir::Class;

    # An instance method
    sub id {
        my $self = shift;
        return $self->{id};
    }

    # Anonymous block: code outside this block cannot see %instance_count
    {
        my %instance_count;

        # Constructor: a class method
        sub new {
            my ($class, $args_ref) = @_;
            ++$instance_count{$class};
            return bless $args_ref => $class;
        }

        # Destructor: an instance method
        sub DESTROY {
            my $self = shift;
            --$instance_count{ref $self};
            return;
        }

        # Another class method
        sub get_instance_count {
            my $class = shift;
            return $instance_count{$class} || 0;
        }
    }

    package MyDir::Class::SubClass;
    use base qw{MyDir::Class};
    # no methods: all functionality is inherited

    package main;

    sub print_all_instance_counts {
        print "MyDir::Class instances:           ",
            MyDir::Class::->get_instance_count();
        print "MyDir::Class::SubClass instances: ",
            MyDir::Class::SubClass::->get_instance_count();
    }

    # Starting counts
    print "*** Counts before any objects are created ***";
    print_all_instance_counts();

    # Create several objects
    my @objs = map { MyDir::Class::->new({id => $_}) } "A" .. "D";
    my @obj_subs = map { MyDir::Class::SubClass::->new({id => $_}) } "W" .. "Z";
    print "*** Counts after initial objects are created ***";
    print_all_instance_counts();

    # Destroy some objects
    undef $objs[-1];
    pop @obj_subs;
    splice @obj_subs, 1, 1;
    shift @obj_subs;
    print "*** Counts after some objects are destroyed ***";
    print_all_instance_counts();
'
*** Counts before any objects are created ***
MyDir::Class instances:           0
MyDir::Class::SubClass instances: 0
*** Counts after initial objects are created ***
MyDir::Class instances:           4
MyDir::Class::SubClass instances: 4
*** Counts after some objects are destroyed ***
MyDir::Class instances:           3
MyDir::Class::SubClass instances: 1