[Exim] modemask option

I'm using exim with a virtual domain setup (in conjunction with
gnu-pop3d and the virtual domain patch), and it has been running very
well. I have password files stored in /virtual/domain.com/passwd
files. Now, in order to allow users to change their passwords, I
made a perl script, however that script obviously needs read/write
access to the passwd files. I can't change the owner of the passwd
file to anything other than 'mail', otherwise exim can't read it.
And I can't give it group read/write access, otherwise exim refuses
to search it and freezes the message. I've tried adding "modemask
002" to my virtual_localuser: directive, but then exim just says
'unexpected "0" after modemask in line 297' In fact, I can't specify
any modemask at all, as far as I can tell. Is there a better way I
should be doing this? Does anyone have a better solution? Thanks
for your help.

Ryan Hayle