caller looks to work sanely inside the Filter::Simple called block (at least with a quick test module).
$ cat FSTest.pm
print STDERR "in FILTER, caller package: ", (caller( 1 )), "\n";
$ perl -MFSTest -e 0
in FILTER, caller package: main
Update: Oop. Quick test module FTL then apparently. You may can get at it by calling caller in a custom import sub of your own, of you might have to fall back to using Filter::Util::Call instead. We're past my simple experience with Filter::Simple at any rate.