[Exim] .forward and external filters

I have exim running to do my mail filtering through the .forward file. I
have a Perl script that I want to run to format the incoming mail. I tried
pipe, but that seems to be more of an external route, and I want something that
will continue on to the next rule in my .forward file.
Is there any such thing? Or would I have to have my external process write the
mail to my inbox?

Thanks,
Jason