[Exim] uid of perl functions

Hi all,

I'm using the following router, calling a special
lookup (get_destinations) to expand the address:

vdom:
        driver = redirect
        allow_fail
        allow_defer
        data = ${perl{get_destinations}}
        file_transport = transport_vdom_file
        qualify_preserve_domain = true
        no_more

permission and owner of the accessed file(s):

drwxrwx--- 3 popowner popowner 512 Oct 25 11:55 ./
drwxrwx--- 4 popowner popowner 512 Aug 1 13:47 ../
-rw-r----- 1 popowner popowner 759 Aug 1 16:32 Accounts

Problem: the perl function seems to work with UID of exim
and therefore is not allowed to read this file.
Setting the options
    user = popowner
    group = popowner
has no effect :(

Any Hint/Workaround would be welcome.

regards
F.