There is little strain on the "language leaders" (that would be barking up the wrong tree, anyway) as the features you named with one exception are already done and have been available for years.

• "is item 'in' set" operator: https://metacpan.org/pod/Syntax::Feature::In

• sets: https://metacpan.org/pod/Set::Object

• clean function parameters declaration system: https://metacpan.org/pod/Kavorka::Manual::Signatures

• constants that are not functions/objects: https://metacpan.org/pod/Const::Fast

• enums that are not functions/objects: ?

• structs that are not functions/objects: pack/unpack, for explanation see https://metacpan.org/pod/Convert::Binary::C