Personal taste - but I'd have the sub first to keep it like map, grep, et al.

sub apply_chain (&$) {
   local $_ = $_[1];
   $_[0]->();
   $_;
}

my $window = apply_chain {
   $_->signal_connect(delete => sub { Gtk->exit(0) });
   $_->set_title("Test");
   $_->border_width(15);
   $_->add( apply_chain {
       $_->signal_connect(clicked => sub { Gtk->exit(0) });
       $_->show;
   } Gtk::Button->new("Quit") );
   $_->show;
} Gtk::Window->new("toplevel");
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