PERL within .procmailrc

I'm trying as an experiment to create a recipe that pipes all incoming
mail to PERL to modify the subject line to add in a list id [naztalk].
Following is what I'm using:

:0 fw: mstst.lock
| /usr/local/bin/perl -e "s/(Subject: )(.*)/\1\[naztalk\] \2/"

However, this isn't working.  Here is the procmail log file for the last
3 tries:

  Folder: Inbox-default                                                       1
  Folder: Inbox-default                                                       1
  Folder: Inbox-default                                                       1

As you can see one byte is being transferred to the Inbox-default folder,
which is the one I use when all the preceeding delivery recipes fail.

Any suggestions?

Glen