Just remove the sub name. Instead of returning a sub reference, you are declaring a sub. Since you aren’t using $env and the return is implicit, just as the the return of the sub is, you can even shorten it to this (again, some style room so do what you like or find most readable)–

sub {
    [
     200,
     [ 'Content-Type' => 'text/plain' ],
     [ "Hello World" ]
    ];
}
[download]

You're just declaring a subroutine, there's no $app variable, which is what plackup is looking for.

Look at your example again:

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

Instead of declaring a subroutine, they are creating an anonymous sub and assigning it to the variable $app.

To make your code work, try the following:

my $app = \&app;

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

This version assigns a reference to the subroutine app to the variable $app, which is the code reference plackup is looking for.

That works. But so does changing the $app line to this–

my $snuffleupagus = \&app;

From the docs–

The last statement of app.psgi should be a code reference that is a PSGI application:

The last statement isn't the sub declaration, it's the assignment. The variable name is meaningless. I personally prefer to just do an anonymous sub { … } as the last or only statement in this kind of test/one-off. But that’s only a style choice.