[Exim] Per user filter

Howdy List,

I'd like to filter certain user's email through a script.
The script will take a message on STDIN and do some stuff then
print either the original message or a modified message to STDOUT be delivered.

If the message is to be dropped it will print nothing.

I also need the account it is being sent to as the argument to the script.
With qmail I can do this for a user in their .qmail file like so:

|myfilter.pl email@??? | safecat ARG ARG

How would one go about accomplishing the same thing in Exim?

TIA

Dan