Well I played with it for a while and this is the closest I could get. Personally I would recommend that you split on the '.' and then iterate over the current list keeping a track of what you have seen already.

print "$_\n" foreach (split(m/\G(([^\.]+)\.\2)|(?:(?!\2\.)\.)/g, "Pugh
+.Pugh.Barney.McGrew.Cuthbert.Dibble.Grub"));
[download]

There's probably some stuff in there I don't need but I'm not a regex master by any means :)