$& is (from perlvar) "The string matched by the last successful pattern match" .. for example s/\d+/foo$&bar/ will give turn 1234 into foo1234bar .. Note (see perlre somewhere) that this can be a significant performance hit ..

As for using /\t\w+/ instead of the look-behind .. i think it makes it easier in some cases .. for (a trivial) example s/(?<=\t)\w+// instead of s/(\t)\w+/$1/ ..