This might not work altogether too well with Gentoo/Alt OSX portage, but it would be a nice idea to check that ._* files are not installed in ${D}; while we use them for etc-update, OSX uses the same naming scheme for data fork files. At least Ruby packages tend to keep these around.
It's called a "resource fork", you might also want to exclude .DS_Store files, which I think you'll find more often in sources than ._<filename> resource forks. Anyway, packages that install those things on non-Mac OS X systems are seriously screwed up. (the .DS_Store file can *always* safely be thrown away)
By the way, the ._<filename> files are actually workarounds to store the resource fork data, that normally the filesystem would cater for. Note that nowadays, resource forks are seldom used anymore. (Instead, on OSX the use of "packages" is now custom, where the finder represents a directory with a given substructure as a file)
You should look at the ruby packages tarballs... no .DS_Store, but lots of ._* to the point I remove them from ${S} in the eclass.
For perl distributions with ._* files I've filed bug 268497
