procmailrc errors from a piped command are considered temporary delivery failures by the mta, and a) are probably requeued, and b) procmail tries to "rescue" the failure, dumping the stream back into the delivery pipeline..

I believe you can add i to the recipe to make it ignore error codes.. so..
:0fwi

I wonder if $mail->reject; needs to have  $mail->exit set to work properly ..?

update - I got it to work with .forward, with contents:
"|exec /home/privbot/bin/filt.pl"
it actually generated a bounce back to the sender.

Notice the pod doesn't mention procmail :) I don't think it works with procmail at all because procmail receives the reject("message") as well as the exit code (constant in Filter.pm) sent by Filter but either tries to recover the "failed" filter and delivers anyway, or depending on what flags are used, just silently stops the pipeline - never does it generate a bounce.. I tried it with many different combinations of procmail flags, even going as far as strace-ing procmail (which revealed nothing except for a successful looking finish).

At least trying .forward with exec should work.. for $mail->reject

I did get all the other methods in Email::Filter to work through procmail though...