In Perl 6 that's probably written like this:

given Gtk2::Window.new( "toplevel" ) {
    .signal_connect( :delete{ Gtk2.main_quit } );
    .set_title( "Test" );
    .border_width( 15 );
    .add( given Gtk2::Button.new( "Quit" ) {
        .signal_connect( :clicked{ Gtk2.main_quit } );
        $_;
    } );
    .show_all;
}
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Though I could see extending the but operator to take a topicalized closure so we don't have to put the ugly $_ at the end:

Gtk2::Window.new( "toplevel" ) but {
    .signal_connect( :delete{ Gtk2.main_quit } );
    .set_title( "Test" );
    .border_width( 15 );
    .add(
        Gtk2::Button.new( "Quit" ) but {
            .signal_connect( :clicked{ Gtk2.main_quit } );
        }
    );
    .show_all;
}
[download]

Merry Christmas!!!