package MyModel;
@MyModel::ISA = qw(SomethingElse Model);

sub changeSomething
{
    my $self = shift;
    my $value = shift;
    $self->{something} = $value;
    $self->_changed('something', $value);
}
package MyView;

sub update
{
    my ($self, $model, $aspect, $otherStuff) = @_;
    # hey! $model has changed its $aspect!
}

package main;

my $view = bless {}, 'MyView';
my $model = bless {}, 'MyModel';

$model->addDependent($view);

# now the following
# calls $view->update($model, 'something', 123);
$model->changeSomething(123);