What For?

• Or design-by-contract

 wrap 'wag', 
 pre => sub { 
 unless (UNIVERSAL::isa($_[0], 'Doggie')) {
 die "wag()'s argument is required to be a Doggie\n";
 }
 unless ($_[0]->is_awake) {
 die "violated precondition for wag():
 Doggie argument is required to be awake\n";
 }
 },
 post => sub {
 unless ($_[0]->consistency_check) {
 die "Doggie $_[0] failed consistency check
 on return from wag()\n";
 }
 };