You know, TinyScheme is essentially in the public domain, and would really be easy to embed into Perl. In fact, you could just use Perl's own scalars as a basic Scheme type and get most of the R5RS spec for free that way...
I think I want to think harder about that. There are a lot of things I want to do in Lisp-y environments without giving up my CPAN, and TinyScheme is really, really lightweight. It's quite conceivable that you could also use the Perl module structure (and CPAN) to organize your Scheme libraries. Hmm...
But even more fascinatingly, a Scheme embedded in XS could also be used to get some introspective (and on-the-fly!) access to other XS-defined items. Say, OLE.XS, which still has me stymied in my effort to get IEMech up and running in the modern age.
So - TinyScheme is probably the lightestweight choice, but there's also Chibi Scheme that looks a more full-featured alternative, plus of course there's already Inline::Guile for serious scheming.
(I'm going to go with TinyScheme, though, and make something self-contained, a la SQLite, that already has everything you need to start with Scheme.)
Oh, here's a link on what hygienic macros are. Makes sense.
Anyway, the motivation for all that is that Sussman physics-in-Scheme course. That thing is incredibly densely written! Fortunately I'm married to a theoretical physicist who, so far, appears impressed at my staying power and is more than happy to talk about anything I find unclear. At length.