stage_id = case stage_id do
  "" -> JobStage.get_job_stage(job_opening_id, 1).id
  _ -> stage_id 
end

I’m currently running 1.8.2-otp-21 and see this behaviour:

iex(1)> defmodule M do
...(1)> def f do
...(1)>   x = :outer
...(1)>   case "" do
...(1)>     "" ->
...(1)>       x = :inner
...(1)>   end
...(1)>   IO.puts x
...(1)> end
...(1)> end
warning: variable "x" is unused (if the variable is not meant to be used, prefix it with an underscore)
  iex:6
iex(2)> M.f
outer
:ok 

As you can see, I see a warning about the “inner” x not beeing used, but I do not see a warning about the imperativ assignment, instead “outer” x stays unchanged.

It would likely generate the following warning:

Note variables defined inside case, cond, fn, if and similar do not leak. If you want to conditionally override an existing variable “stage_id”, you will have to explicitly return the variable.