I see it prints HTTP headers with the status line first. So I changed the handler to work around that. I capture the output that has been written through CGI STDOUT, then mangle the headers and print that.

sub handler {
    capture {
        MyModule->new->go;
    } \$out;

    my ($header, $body) = split /\r\n\r\n/, $out, 2;
    my $h = HTTP::Headers->new;
    for my $line (split /\r\n/, $header) {
        my ($key, $value) = split /: /, $line, 2;
        $h->header($key, $value);
    }
    $h->header('Status' => 200) unless $h->header('Status');
    my $status = $h->header('Status');
    $h->remove_header('Status');
    print 'HTTP/1.1 '
      . HTTP::Response->new($status => undef, $h, $body)->as_string("\
+r\n");
}
[download]

Now the tests pass, except the last one! That's the one about POST data. Thus:

is $r->content, $post_data;
#   Failed test at foobar.t line 85.
#          got: ''
#     expected: 'some post data'
# Looks like you failed 1 test of 8.
[download]

diag $r->as_string shows:

# HTTP/1.1 201 Created
# Location: http://localhost:12345
# Content-Type: text/html; charset=ISO-8859-1
# Client-Aborted: die
# Client-Date: Tue, 17 Feb 2009 18:16:12 GMT
# Client-Response-Num: 1
# X-Died: read failed: Connection reset by peer at .../LWP/Protocol/ht
+tp.pm line 382, <DATA> line 16.
#
[download]

Note that the transmitted HTTP body disappeared. I can confirm that it does not arrive anymore by the time CGI is initialised. I can confirm that the capturing causes that.

What do I do now?