package MyClass;

# fully functional constructor method
sub new {
  # if we're trying to instantiate the abstract class, die
  my $class = shift;
  die "abstract class must be subclassed" if $class eq 'MyClass';
  
  # do some processing, all with lexically scoped variables
  # ...
  
  # call (as of yet) unimplemented function to return a hashref
  my $self = _underthehood();
  die "_underthehood() not implemented" if ref $self ne 'HASH';
  
  # bless hashref as instance of the current package, and return it
  bless $self, $class;
  return $self;
}

# unimplemented member method
sub _underthehood {
  return;
}

1;

##</code><code>##

package MyClass::Implemented;

use base 'MyClass';

# newly implemented member method
sub _underthehood {
  # do some processing on the given arguments,
  #creating a populated hashref
  my $hashref = { 'key' => 'value' };
  
  return $hashref;
}

1;

##</code><code>##

#!/usr/bin/perl -w

use strict;
use MyClass::Implemented;

my $obj = new MyClass::Implemented();