The problem to add all objects to a stack (your @pack array) is that this will break Object destruction, since you always will have a reference to any object in the @pack list. To avoid that you can use the module Hash::NoRef, where you can add values to this hash without count the references to this values, keeping the behavior of the garbage colection system.

package Dog ;

  use Hash::NoRef ;

  use vars qw(%GHOST_TABLE) ;
  tie(%GHOST_TABLE , 'Hash::NoRef') ;

  my $GHOST_ID ;
  
  sub new {
    my $class = shift ;
    my $this = bless( {} , $class ) ;

    ++$GHOST_ID ;
    $GHOST_TABLE->{$GHOST_ID} = $this ;

    return $this
  }
  
package main ;

{
  my $obj = new Dog() ;

  print $GHOST_TABLE->{1} ; ## will show the object reference
}

  print $GHOST_TABLE->{1} ; ## will show undef
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