Sorry, one steap ahead of you again. :-) That breaks down when you need to add a nameless widget to another also nameless widget:

for (HBox->new) {
  $_->config1($param1);
  $_->config2($param2);
  my $sb = Scrollbar->new;
  for ($sb) {
    $_->config1($param3);
    $_->config2($param4);
  }
  $_->add($sb);
  $window->add($_);
}
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You need a temporary variable, although at least this one can be tightly scoped.

It also mingles method calls against several different widgets into a single block, which makes it harder to see what gets added where when. Your proposal is ok on a smaller scale, but scales badly - if you nest several unnamed widgets, it gets messy quick.

If you look at the tutorial at GtkPerl.org you'll see a really ugly style with lots of variable reuse and calls to different widgets interwoven in a single block. It made my head swim trying to read even the shorter example programs. configure_object was the result of several iterations of refactoring for readability.

It took enough effort and was beneficial enough that I felt I had to share this.

Update: Actually I suppose one can coerce the for approach into clean separation by adding some do to the mix:

$window->add(do {
  my $hbox = HBox->new;
  for ($hbox) {
    $_->config1($param1);
    $_->config2($param2);
    $_->add(do {
      my $sb = Scrollbar->new;
      for ($sb) {
        $_->config1($param3);
        $_->config2($param4);
      }
      $sb;
    });
  $hbox;
});
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but that's quickly slipping into the realm of cumbersome. I really spent some serious time trying to get this whole deal to look reasonably readable, and no conventional tools would help, hence the snippet.

Makeshifts last the longest.