Hey guys, modsec-rules-updater needs LWP::UserAgent which is provided by dev-perl/libwww-perl. Tested with www-apache/mod_security-2.5.9.
Good catch, I'll check in a bit, I'd sincerely put it under a perl USE flag, since I don't really want to have perl stuff on my systems, and I don't use the updater anyway...
Fixed in 2.5.9-r1 via perl USE flag.