my $button = $mw -> Button(-text => "Quit", 
        -command => sub { exit })
    -> pack();
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Arrgh. Don't use a bare exit; in a Tk program, use Tk::exit; It won't matter in this little snippet, but for a larger more complex GUI with multiple windows/widgets Tk::exit; will do a controlled cleanup and shutdown and avoid all kinds of warnings about unreleased resources.