[Exim] looking for detailed docs on embedded Perl

I've spent many hours searching for detailed notes on using the Embedded
Perl functionality of Exim v3 - I've only found the most general examples.

Specifically, I have a Perl script that I wish to use within a FILTER, but
it needs to access a DBM file. Both tie and dmbopen() cause the following
error:

error in filter file: failed to expand "${perl{isSpam}}\n" in logwrite
command: Undefined subroutine &main::isSpam called.

Commenting out the DBM stuff causes the script to run error-free. The
embedded Perl thing has a lot of potential, but I'd sure like a list of
limitations.....

-many thanks
Mike Scott