I'm not sure we've reached consensus here, so I will try to summarize
what everyone said so far in order to clear my own head a bit. :)
Sorry in advance if i horribly misrepresent anyone's opinions.
Luke: Thinks the "_" syntax is no joke, since every language with
pattern matching abilities has it. Further, using "undef" won't work
because it means "this has to be undef" in signature unification.
Dks: Replies to Luke. Thinks that "undef" is more perl5ish, and that
"_" can disappear too easily, and is too Huffmanized. Doesn't think
"_" is obvious, but is sure we will end up using it if this feature is
TSa: Prefers to rely on lazy evaluation, and says both tounge-in-cheek
and philosophically that if I don't want to care about some elements,
I should do so, and let Perl6 optimize. Proposes several ways of not
giving a name to a variable.
Yuval Kogman: Replies to David. Seems to repeat Luke's point about
value binding in signatures. Shows an example of value binding in
action. Argues for the "_" or "__" syntax. Thinks that making this
feature easy encourages reuse of calling code, not only callee code.
So, in summary, it's not obvious that this feature is needed at all
because things can be optimized away. Also, neither the "undef" nor
the "_" syntax have 100% support, and "undef" seems to have problems
related to signature unification.
Hm. For the time being I will assume that this functionality is not
sufficiently wanted to deserve its own syntax. But I must admit I'm
actually starting to like "_" in subroutine signatures... not sure if
it works well with positionals and the like, however.