package Foo;
use Carp;

# Import method that checks existence of abstract methods in subclasses.
sub import {
  my $pkg = shift;
  return if $pkg eq __PACKAGE__;
  foreach my $meth ( qw(foo) ) {
    $pkg->can($meth) or croak("Class $pkg does not define method $meth");
  }
  $pkg->SUPER::import(@_);
}

1;

##</code><code>##

package Bar;
use Carp;
use Foo;
@ISA = qw(Foo);

# Import method that checks existence of abstract methods in subclasses.
sub import {
  my $pkg = shift;
  return if $pkg eq __PACKAGE__;
  foreach my $meth ( qw(bar) ) {
    $pkg->can($meth) or croak("Class $pkg does not define method $meth");
  }
  $pkg->SUPER::import(@_);
}

1;

##</code><code>##

package Baz;
use Bar;
@ISA = qw(Bar);

sub foo {print "This is method foo\n";}

sub bar {print "This is method bar\n";}

1;

##</code><code>##

use Baz;