When emerging git with USE="subversion -perl", what happens is that subversion is not a dependency, but built_with_use dev-util/subversion perl is called in pkg_postinst(), resulting in an error if subversion is not installed. With USE="subversion -perl", the most reasonable action would be to error immediately, informing the user that USE=perl needs to be specified as well. It's non-obvious that git-svn requires perl after all (from a user's perspective).
Fixed in CVS.