I thought I should update this based on what we learned post-mortem, in case anyone should stumble into this problem again: Our web server setup used to be on the same machine as the rest of the system. Then it moved to another machine, and we NFS-mounted the code base (read-only). Apparently, Apache::Cache creates some kind of temp file in a default location (apparently, because I haven't found the exact documentation of this and I'm guessing), which appears to be the same directory as the code which uses Apache::Cache. If it can't create this file, it chokes. The file remained there after we changed the directory to be read-only NFS-mounted (why it wasn't deleted when the web server exited, I dunno). Even though it was read-only, Apache::Cache was happy to see it, although it (apparently; boy, I hate having to use that word so much) never tried to write to it (which would have caused a different chokeage, I guess). As it turns out, we were rethinking our decision to use the NFS mount, because it meant our webserver would be unusable if the main machine had to go down. So we undid all that and use rsync to keep the source directory updated. Once we made the directory writeable, the problem vanished.