The encryption extension is no Shared Object (not an .so file, but an .a file, and running it with a Perl executable different from that one provided with the extension ends up in 'Can't find shared object file for ... in @INC'). So it seems it is not possible to use this extension with a patched version of Perl to get the code. But I'm a C beginner, perhaps there IS a way?

Wait, does that mean a custom Perl is needed? If so, you're not talking about the strength of a filter.