Given that two respected Perl gurus both thought that this would break, here is a working test with three modules. Foo and Bar are abstract. Baz must implement their abstract methods.

Here is Foo.pm:

package Foo;
use Carp;

# Import method that checks existence of abstract methods in subclasse
+s.
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;
[download]

Here is Bar.pm:

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

# Import method that checks existence of abstract methods in subclasse
+s.
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;
[download]

Here is Baz.pm:

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

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

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

1;
[download]

And then here is the test script:

use Baz;
[download]

If you comment out either sub in Baz.pm, the program will die immediately.

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