runs mount of /bin, /usr/bin, /lib, and /tmp/build into /tmp/install/bin,

Nice! I'll try again to install Google Earth for linux using that trick. Silly me.

My problem is that I want to build mod_perl and this [...] b)wants to install things into the Perl library directories.

UnionFS comes to mind. It allows you to mount your whole filesystem (including /tmp/build) under /tmp/install read-only, then mount an empty filesystem also in /tmp/install. Do chroot; make install and UnionFS will take care of committing changes into the empty filesystem, without touching / (since it is read-only). This is how CD-based distros allow users to "write" to the CD.