A lot of your issues can be resolved by using the :common attribute. For example:

class Foo {
  field $x :param = 0;
  field $y :param = 0;

  method create :common ( %args ) {
    if ( $args{x} < 0 ) {
      return undef;
    }
    else {
      return $class->new( %args );
    }
  }
}

my $foo = Foo->create( x => $x, y => $y ) or do_some_error_handling();
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Note that this is supported in Object::Pad but not in the first round of implementation for the class keyword (likely to be included in Perl 5.38). As Object::Pad is the testing ground for the class keyword, we can hopefully take it as an indication that :common will be supported in a forthcoming version of Perl, hopefully Perl 5.40.

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