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MIYAGAWA / Plack-Middleware-Debug-0.16 / examples / hello-world / app.psgi 
use Plack::Builder;

my $app = sub {
    return [ 200, [ 'Content-Type' => 'text/html' ], [ '<body>Hello World</body>' ] ];
};

builder {
    enable 'Debug';
    $app;
};