Safely executing your mail filter?

So all incoming mail that comes to my domain gets put via qmail into a 
perl script and custom filtering is performed.

Occasionally an error makes it's way into the perlscript and it fails 
to run. When this happens I lose email.

I would like to ensure that this doesn't happen.

Is there any way to safely execute a perlscript or should I be looking 
to wrap it via a shellscript that checks the exit status to decide 
whether it can trust the script or resort to another approach.

Currently my $HOME/.qmail script looks like this

bash$  cat .qmail
| /home/robert/perlscripts/filter.pl

I would like to change that so that it uses a shell script which is 
never changed that wraps my perl script and based on it's exit status 
will manage to catch all incoming mail that should have been filtered 
into my INBOX.

How, can I do that? Specifically with qmail ... what happens to email 
that does _not_ make it's way to my mail audit's "accept" calls. Is 
there any way for this email to go into the general inbox?