[eluser]pickupman[/eluser]
I was on 1.7.2 when I posted the snippet with the same error as you. You can change FCPATH definition. Searching the source, it is only referenced in system/libraries/URI.php @line 149. Shouldn't break anything. Or just add the snippet I posted into your carabiner.php file to check and strip it.
Profile your code, and see which way is fastest. Running my preg_match every time maybe more overhead than editing FCPATH correctly, or using dirname().