Insane writer?

gistfile1.txt

#!/usr/bin/env perl

use strict;

use Plack::Builder;
use Plack::Middleware::ErrorDocument;

my $root   = '/home/leo/git/fab/vhosts/ah/docs/www';

my $app = sub {
    sub {
        my $respond = shift;
        my $writer = $respond->([404, ['Content-Type' => 'text/html', 'Content-Length' => 3]]);
        if (!ref($writer) || !eval {$writer->can("write") and $writer->can("close")}) {
            die("writer is insane!");
        }
        else {
            $writer->write('foo');
            $writer->close;
        }
    };
};

$app = Plack::Middleware::ErrorDocument->wrap(    #
    $app,
    404 => "$root/page_not_found.html",
);

my $app = builder {
    $app;
};

$app;                                                 # return

Plack: 0.9986
Moose: 2.0402 (all moose modules also updated)
Catalyst: 5.90011
Starman: 0.300

Discussing on #catalyst (which is where the $app sub came from)

11:16 @t0m Ranguard: yes, and it's something to do with the subrequest, I think
11:19 @t0m Ranguard: ah, ok, I think I just understood..
11:19 @t0m Sooo, what happens is..
11:19 @t0m Catalyst app gets called, returns 404
11:20 @t0m ErrorDocument gets called, substitutes the response with the opened FH for the error file to serve...
11:20 @t0m Catalyst then gets passed back it's body writer
11:20 @t0m which errordocument has just scribbled on
11:20 @t0m Catalyst throws a fit..
11:21 @t0m I think this is a bug in Plack::Util::response_cb

ErrorDocument's subrequest was a bad idea and doesn't support streaming.