/usr/portage/dev-util/yacc/yacc-1.9.1-r1.ebuild fails because the distribution tar file isn't properly extracted. A workaround is to add "tar xvf ${P}.tar" after the invocation of unpack in src_unpack().
However, the real problem appears to be the unpack function. Near line 185 of ebuild.sh: y="$(echo $x | sed 's:.*\.\(tar\)\.[a-z0-9]*:\1:')" should read: y="$(echo $x | sed 's:.*\.\(tar\)\.[A-Za-z0-9]*:\1:')"
Will be fixed in Portage 2.0.27