Well, pack/unpack has its own limitations. The template might be too cumbersome even if it works. A good, expressive template is usable both for pack() and unpack(). You've already gone into hacks territory with those Xx-s (to unpack a byte string not immediately preceded by its length).

If you want to discard some value, the usual way is to

my ($foo, undef, $bar, $baz) = unpack ...;