Missing Plack body filter feature?

app.psgi

#!perl
use Plack::Builder;
builder {
	enable sub {
		my $app = shift;
		sub {
			my $env = shift;
			Plack::Util::response_cb( $app->( $env ), sub {
				my @out = ( 'a' .. 'z' );
				sub { shift @out }
			} );
		}
	};
	sub { [
		200,
		[ "Content-Type", "text/plain" ],
		[ ( "Hello World\n" ) x ( ( 0 + $_[0]{'QUERY_STRING'} ) || 1 ) ]
	] };
};