... preamble as before
use strict;

my $_count = 0;      # count of instances created

sub new {
    my $package = shift;
    my $self = SUPER::new(@_);

    # The class we inherit from counts its instances.
    # Since we're really creating one of us, adjust
    # its count downward, then point the object at
    # our counter.
    ${$self->{_count}}--;
    $_count++;
    $self->{_count} = \$_count;

    bless $self, $package;
}