File transfer with Apache2::Upload and XMLHttpRequest

 I tried to switch from the 'common' way to send a file to the server, which is in my case:

a) define a form with an file-input-element:

<form method="post" action="gallery?action=upload_image" enctype="multipart/form-data">
    <input type="file" name="file"/>
    <input type="submit" value="Submit" />
</form>

b) define mod_perl code, that saves the file-data inside a handler, something like:

    my $apr = Apache2::Request->new($r);
    my $upload = $apr->upload('file');
    ....

That works fine as it is easy to work with $upload and save the data.
But I really can not figure out how to handle the XHR-Approach on the server-side because now, of cause, the 'file' param is missing.

The JS client side code is (where 'file' is a HTML5 File-Object):

var xhr = new XMLHttpRequest();
xhr.open("POST", url + "?filename=" + encodeURIComponent(file.name), true);
xhr.setRequestHeader("Content-Type", "multipart/form-data");
xhr.send(file);

Any idea how now the server side part has to look like now?

Thanks, Michael