sub default_code {
    my ($self,$extra) = @_;
    my $sub = sub { 
        my ($extra,$optional,$optional) = @_;
        # do something
        };
    return $sub;
}

##</code><code>##

$object->display( { my_code => sub { 
                        my ($hashref) = shift;
                        foreach (keys %$hashref) {
                            # more code 
                        }
                             }; 
                   }
                 );

##</code><code>##

sub display {
   my ($self,$args) = @_;
   # $args->{'my_code'} has our "custom code"
   # here is where we determine which one to use
   my $action = $args->{'my_code'} || $self->default_code;
   $action->($args);
}