It’s best to use some form of {module, function, args} or &Module.function(&1, arg1, arg2.., argN)¹.

See here and here.

So doing something like

fn x -> 
  {y, z} = fabulate(x)
  send(y, z)  
end

would cause problems, but, on the other hand:

A fully qualified fun (fun Mod:Fun/Arity) will however be a fully
qualified function that doesn’t refer to any specific code version and
will be reloadable.

And here’s one more from Jose Valim: https://github.com/elixir-lang/elixir/issues/6003#issuecomment-294376459

I guess you could get away with it if you’re careful, but I’d rather play it safe and use {module, function, args}

¹ I haven’t verified if this form will survive a code reload.