Outputting HTML in App is causing "Cannot modify header information" errors

Hi,

I've built a small Perch app to skin the admin, which outputs CSS and some other things in the head using the Perch app API.

It works great but I have a problem when trying to echo out admin favicon, getting a "Cannot modify header information"… error. I know this error is usually caused by things outputting in the wrong place. I assume it's because I'm just echo'ing it out rather than using an official API method.

    $Perch = Perch::fetch();
    $Perch->add_css($API->app_path() . '/standard-admin.css');
    echo '<link rel="shortcut icon" href="/perch/addons/plugins/ui/favicon.ico">'; <-- This causes an error

How would I echo this out correctly? I did try looking through the API documentation but couldn't piece anything together—either that or I'm not looking correctly. Incidentally, although I was told about $Perch->add_css through the forum, I could not find this documented and I'm curious where I would find this info. I'd like to be able to troubleshoot this on my own but either it's not documented or I can't find it.