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

##</code><code>##

my $app = \&app;

sub app {
     my $env = shift;
     return [
         '200',
         [ 'Content-Type' => 'text/plain' ],
         [ "Hello World" ]
     ];
 }