This just hit me as The Right Way To Do This. 
package Foo;
use Carp;
# Time passes

# 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 bar) ) {
    $pkg->can($meth) or croak("Class $pkg does not define method $meth
+");
  }
  $pkg->SUPER::import(@_);
}

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The import method will do nothing if you import the base class, but will die a horrible flaming death if you ever use or import a class that inherits from it which does not implement the abstract methods you want.

Note that the base class cannot (with this method) define the abstract methods itself, it just lists them. (They could be in an array, etc.)

UPDATE
I added inheritance so one abstract class can inherit from another. This breaks on multiple inheritance because Perl's SUPER mechanism doesn't handle this cleanly.

UPDATE 2
tye pointed out in the chatter that I should document the fact that if you define your own import method and don't call the SUPER::import method inside of it, then you will break this mechanism.

UPDATE 3
I took the snippet above and turned it into a useful module which may be found at AbstractClass.

In reply to Abstract class methods by tilly
in thread Interfaces in Perl? by gregorovius

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