Very smart! That's exactly what I'm looking for! Thanks! But the "after BUILD" part does not work, it complains about

The method 'BUILD' was not found in the inheritance hierarchy for Mypackage at /usr/lib/perl5/Class/MOP/Class.pm line 1053

So I changed to :

sub BUILD{
  $newest_instance = shift;
}
[download]

Then it works. may be I miss some perl modules...