package Foo;
  our %callbacks = (
      pig => sub { ... },
      cat => sub { ... },
      dog => sub { ... }
  );
  
  while (...) {
      # do stuff
      $choice = ...;
      $callbacks{$choice}->(@args);
  }

##</code><code>##

  package main;
  use Foo;
  $Foo::callbacks{iguana} = sub { ... };

##</code><code>##

%Time ExclSec CumulS #Calls sec/call Csec/c  Name
90.73   0.770  0.648  60783   0.0000 0.0000  Foo::__ANON__

##</code><code>##

  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 { ... }