The main motivation is not to save one keystroke (that just ends up happening because "_" is one keystroke fewer than "::"). The main motivation is dependency injection. When you do MyApp::Widget->new() you are hard-coding the class name. Hard-coding is code smell. There are many Perl programmers who would agree that hard-coding is bad, but don't even think of using class names like that as hard-coding. But it is hard-coding and it makes your code less adaptable. The idea in Zydeco is instead of doing MyApp::Widget->new(), you do $self->FACTORY->new_widget(). Now the widget class isn't hard-coded, and if you need to use a different widget class (such as a mock widget for your test suite) you can just wrap new_widget.

package MyApp {
  use Zydeco declare => [qw(TestingRole)];
  
  my $testing = true;
  
  role TestingRole {
    ...;
  }
  
  class Widget {
    ...;
  }
  
  if ($testing) {
    around new_widget (@args) {
      my $widget = $self->$next(@args);
      Moo::Role->apply_roles_to_object($widget, TestingRole->role);
      return $widget;
    }
  }
}

my $factory = 'MyApp';

print $factory->new_widget, "\n";
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(Note that the declare bit it only so we can declare TestingRole as a bareword for use later on. In general you don't need to declare the existence of roles and classes beforehand.)