A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

use strict;
 
package Delegator;
sub new {
   bless {}
}
sub operation {
   my ($self) = @_;
   if (defined $self->{delegate} && $self->{delegate}->can('thing')) {
      $self->{delegate}->thing;
   } else {
      'default implementation';
   }
}
1;
 
package Delegate;
sub new {
   bless {};
}
sub thing {
   'delegate implementation'
}
1;
 
 
package main;
# No delegate
my $a = Delegator->new;
$a->operation eq 'default implementation' or die;
 
# With a delegate that does not implement "thing"
$a->{delegate} = 'A delegate may be any object';
$a->operation eq 'default implementation' or die;
 
# With delegate that implements "thing"
$a->{delegate} = Delegate->new;
$a->operation eq 'delegate implementation' or die;
 