package Foo;
  sub Foo::callbacks::pig { ... } # no more hash!
  sub Foo::callbacks::cat { ... }
  sub Foo::callbacks::dog { ... }

  while (...) {
      # do stuff
      $choice = ...; # of course, we also validate $choice
      no strict 'refs';
      &{"Foo::callbacks::$choice"}(@args); # I feel so dirty!
  }
  
  package main;  
  ## now extend Foo by doing this:
  use Foo;
  sub Foo::callbacks::iguana { ... }