In case someone else has a similar problem, I’m reporting that, having revisited it more recently, I seem to have solved the issue of the shader that wouldn’t compile.
Simply put, it looks as though the problem was that “l_normal”–the normal vector that is an output of the vertex shader and an input to the fragment shader–had no semantic assigned to it. This doesn’t seem to have been a problem in 1.8, but I presume that 1.9 (or some third-party element included therein) is stricter about these things.
Interestingly, the shader only fails to compile when the semantic is omitted from the fragment shader; it compiles happily when the semantic is included in the fragment shader but omitted from the vertex shader, although it produces incorrect results in this case.
(And yes, I do feel a little silly for not having previously tried correcting this omission. ^^; )