Just in case anyone else runs into this problem, installing LWP on a Mac apparently overwrites /usr/bin/head due to the case insensitivity of HFS+. This is used by Apache during the build process, so it can go unnoticed until the next time you're building Apache, when you see this:

(Followed by an extensive usage message and repeated several times.)

To fix it, restore /usr/bin/head, and you're good to go.