[Exim] perl for normal

I have a perl string expansion working fine in my system filter.

But it seems I can't use any perl string expansion in normal user's filter.

Here is the error

== someone@??? D=userforward defer (-11): error in filter file:
failed
to expand "${perl{some_code}{$input1}{$input2}}" in filter file:
missing or misplaced { or }

Any suggestion??

Sam Ng