Emerge command strips executables, damaging upx-compressed files (upx itself). I just copied manually the executable and worked fine. Forum people suggested to add: RESTRICT="nostrip" to the ebuild.
I added upx-1.23-r1 which should not strip the upx binary. Please test it out and let me know if things work properly now. It is currently marked unstable.
And now I bumped it to version 1.24
I tested both 1.23-r1 and 1.24 ... they work fine; thanks !
upx-1.24 has been marked stable in portage.