The purpose of an abstract class is to constrain the implementor of its subclasses. This being perl, we don't try too hard to prevent circumvention of our constraints. So a very simple implementation of an abstract class is:

  package BaseClass;

  sub new {
    my ($class, @args) = @_;
    my $self = bless {}, $class;

    $self->init(@args);
    $self->check_interface__BaseClass;
    $self;
  }

  sub init { die "oops, this is the base class" }

  sub check_interface__BaseClass
  {
    my ($self) = @_;
    foreach my $method (qw[ foo bar baz ])
    {
      $self->can($method) or die "method $method not implemented";
    }
  }
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With this simple approach, if your subclasses don't redefine "new" (they define the init method instead), then they get the interface checking. If they do redefine "new", then they can still check the interface by calling the check_interface method explicitly.

Sure, it has its limitations, but if you want a lightweight solution, this seems to fit the bill. --Dave