Common attribute idioms

Complex default values.

• default values are either ...
• non-references (numbers, strings)
• subroutine references

has 'foo' => (
    is      => 'rw', 
    isa     => 'ArrayRef', 
    default => sub { [] }
);

• default sub is passed $self

 has 'obj_id' => (
     is      => 'rw', 
     default => sub { refaddr($_[0]) }
 );