It sounds like you are trying to store two different objects in the same object. Instead of having child override anything, give it an additional $self->{mom} attribute that is its own person.

A really horrible way to fix it would be to have child::introduce do

   my $oldclass = ref $self;
   bless $self, "person";
   print "Here's my mom: ";
   $self->introduce();
   bless $self, $oldclass;
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