First off, if you know enough to go the XS route, you probably also know enough to go the command-line-tool route, using the same libraries you have already found - I'd suggest that you write a simple wrapper in C++ and allow it to take text-based arguments plus a picture to recognize.

As for a pure-perl option, I'm not sure you're going to want to go that route, though I don't know what kind of hardware you're working with. The reason is that it will very likely be slow, unless you use some kind of semi-compiled language extension to do the work, such as PDL.

PDL may be the kind of thing you are looking for. It seems likely that someone's written facial recognition (or other biometric identification) software in PDL.

Good luck,
- D