Look no further than the bible of OO in Perl: Object Oriented Perl describes how to implement this on page 185-186.

The basic idea is to set all the methods in the abstract class so that they die (or croak) if called. Of course you may want one more level of abstraction and write a generic method that will do this, and just have any other method call it.

I've done this in the past, using code like the following:

package Base;

use Carp;

my @methods = qw(new something something_else);
my %methods;
@method{@methods} = ();

sub AUTOLOAD {
  my ($pkg, $method) = $AUTOLOAD =~ /^(.*)::(.*)$/;

  if (exists $method{$method}) {
    croak qq(Package "$pkg" must override the method "$method");
  }
}

1;
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If you then define a subclass that uses the Base.pm interface, you get an error if you call an undefined method:

#!/usr/bin/perl -w

use strict;

package MyObj;

use vars qw(@ISA);
use Base;

@ISA = qw(Base);

sub new {
  return bless {}, shift;
}

package main;

my $thing = MyObj->new;

$thing->something;
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Of course, this only generates the error if a missing method is called. It's probably possible to do a more stringent check when the derived class is loaded. I'll give that some thought...

"The first rule of Perl club is you don't talk about Perl club."