If you're working against Elixir v1.1, here's a simple (single tier) with statement

poor_persons_with.exs

defmodule MyModule do
  defmacro exec_with(left, right, do: fun) do
    quote do
      case unquote(right) do
        unquote(left) ->
          unquote(fun)
        v -> v
      end
    end
  end
end

iex(1)> defmodule MyModule do
...(1)>   defmacro exec_with(left, right, do: fun) do
...(1)>     quote do
...(1)>       case unquote(right) do
...(1)>         unquote(left) ->
...(1)>           unquote(fun)
...(1)>         v -> v
...(1)>       end
...(1)>     end
...(1)>   end
...(1)> end
iex(2)>
iex(3)> import MyModule
iex(4)>
iex(5)> val = exec_with { a, b, c }, { 1, 2, 3 } do
...(5)>     a + b + c
...(5)> end
iex(6)> val == 6
iex(7)>
iex(8)> val = exec_with { a, b, c }, 1000 do
...(8)>     a + b + c
...(8)> end
...(8)> val == 1000
iex(9)>
iex(10)> val = exec_with { a, b, c } = res, { 1, 2, 3 } do
...(10)>     res
...(10)> end
iex(11)> val == { 1, 2, 3 }
iex(12)>
iex(13)> val = exec_with { a, b, c }, { 1, 2, 3 }, do: a + b + c
iex(14)> val == 6