package DuckInterface;
use Carp;

my @inheritors;

sub import {
  my $caller = caller; 
  push @inheritors, $caller;
}

  my @abstract_methods = qw(swim fly);

  sub INIT {
    my $bad = 0;
    for my $class (@inheritors) {
      for my $meth (@abstract_methods) {
        no strict 'refs';
        unless (defined &{"${class}::$meth"}) {
          $bad=1;
          warn "Class $class should implement DuckInterface, but does not define $meth.\n";
        }
      }
    }
    croak "Compilation aborted" if $bad;
  }

  1;