That's a nice & small answer. It does seem overkill for the gtk example...
use v6; use GTK::Simple; with GTK::Simple::App.new(title => 'abc') { my $app = $_; my $button; .set-content: GTK::Simple::VBox.new( GTK::Simple::Button.new(label => 'bcd').clicked.tap({ .sensitive = False; $button.sensitive = True; }); $button = GTK::Simple::Button.new(label => 'cde'); $button.clicked.tap({ $app.exit; }); ); .border-width = 20; .run; };