Provding defaults

• if default is a ref, always wrap it in a sub{ ... }
• consider:

has array => default => []; # ARRAY(0x123456)
my $class  = Class->new;
$class->array; # ARRAY(0x1234567)
my $class2 = Class->new;
$class2->array; # ARRAY(0x1234567)
$class->array->[0] = 42;
$class2->array->[0] # now it's 42!

• this is never what you want, so Moose dies if you try to do this

sub mk_array { [] };
mk_array; # ARRAY(0x1234567)
mk_array; # ARRAY(0x6543210)