Exit either by clicking the button or the close window control in the upper corner.
use GTK::Simple;
use GTK::Simple::App;
my GTK::Simple::App $app .= new(title => 'Simple GTK Window');
$app.size-request(250, 100);
$app.set-content(
GTK::Simple::VBox.new(
my $button = GTK::Simple::Button.new(label => 'Exit'),
)
);
$app.border-width = 40;
$button.clicked.tap: { $app.exit }
$app.run;