One thing we didn't do in this chapter was run Devel::Peek on a subroutine. Try it on a named subroutine reference, an anonymous subref and a subref to an XS routine.
See if you can work out what 'FM', 'IO', 'BM' and 'PVLV' are in the above; try creating them in Perl and dumping them out with Devel::Peek. Use sv.h to explain the new fields.