EPREFIX seems wrongly expanded, so when it is undefined the / gets eaten.
http://sources.gentoo.org/gentoo-projects/portage-utils/main.h?r1=1.11&r2=1.12
*** Bug 394583 has been marked as a duplicate of this bug. ***
*** Bug 394631 has been marked as a duplicate of this bug. ***
The fix seems still not in Portage at Dec 19 00:00 UTC 2011.
*** Bug 395221 has been marked as a duplicate of this bug. ***
*** Bug 395315 has been marked as a duplicate of this bug. ***