caller looks to work sanely inside the Filter::Simple called block (at least with a quick test module).

$ cat FSTest.pm
package FSTest;

use Filter::Simple;

FILTER {
  print STDERR "in FILTER, caller package: ", (caller( 1 ))[0], "\n";
}

1;
$ perl -MFSTest -e 0
in FILTER, caller package: main
[download]

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.