package Abstract;
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 inherits from Abstract, but does not define $meth.\n";
      }
    }
  }
  croak "Compilation aborted" if $bad;
}

1;

##</code><code>##

Class Fish inherits from Abstract, but does not define fly.
Compilation aborted at fish.pl line 0

##</code><code>##

package Fish;
use Abstract;

sub swim {
  "bloop bloop bloop";
}

1;