Currently =virtual/perl-MIME-Base64-3.07 is keyworded ~sparc-fbsd. (The virtual rdepends on dev-lang/perl-5.8.8 or perl-core/MIME-Base64-3.07.) As soon as =virtual/perl-MIME-Base64-3.08 and =perl-core/MIME-Base64-3.08 were added to the tree, your keyword was dropped, because this virtual rdepends only on perl-core/MIME-Base64-3.08 which isn't keyworded.
done for 3.09 -> fixed