can you express that grammar in ebnf? I think the parser would need to be at least ll(C) where C is bigger than 1.

Would the presence of $_ denote a lambda? how would nested lambda's work?

Why not:

    put 1 2 3 4 5 | filter -> > $1 2 | map -> * 2 $1

I think that would be easier to parse, and not much typing overhead, also $_ is a weird thing.