You can find the answer to this exercise in "Answer for Chapter 13" in the Appendix.
13.8.1. Exercise [45 min]
Modify the RaceHorse class to get the previous standings from a DBM hash (keyed by the horse's name) when the horse is created, and update the standings when the horse is destroyed. For example, running this program four times:
my $runner = RaceHorse->named('Billy Boy'); $runner->won; print $runner->name, ' has standings ', $runner->standings, ".\n";
should show four additional wins. Make sure that a RaceHorse still does everything a normal Horse does otherwise.
For simplicity, use four space-separated integers for the value in the DBM hash.