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

sub Foo::create ( $class, %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();