Thanx to runrig, who pointed out that $a and $b are package-scoped, not global-scoped, I modified the sort function to:

my $sortfunc = sub { $efgh::a->{blah} cmp $efgh::b->{blah} };

That not only fixed the simple example (where it wasn't sorting), but it also fixed the original app. I have no idea why it was complaining about pseudo-hashes, but I have a working script+module now. :)