Enabling Filters Causes a Header Error

When I enable filters, adding any filter to the /addons/templates/filters.php file Perch outputs the contents of the filters.php file at the top of the document, then returns errors:

Warning: Cannot modify header information - headers already sent by (output started at /path-to-site/perch/addons/templates/filters.php:1) in /path-to-site/perch/core/lib/PerchUtil.class.php on line 1399

Warning: Cannot modify header information - headers already sent by (output started at/path-to-site/perch/addons/templates/filters.php:1) in/path-to-site/perch/core/lib/PerchUtil.class.php on line 1400

Warning: Cannot modify header information - headers already sent by (output started at /path-to-site/perch/addons/templates/filters.php:1) in /path-to-site/perch/core/lib/PerchUtil.class.php on line 1401

Warning: Cannot modify header information - headers already sent by (output started at /path-to-site/perch/addons/templates/filters.php:1) in /path-to-site/perch/core/lib/PerchUtil.class.php on line 1409

I've seen other support tickets about whitespace before the runtime, there is no whitespace preceding the runtime call.

Can anyone shed any light on this?