Inside the processed template:
  x1 = x1_defined_in_template
  x2 = x2_defined_in_stash_set
  x3 = x3_defined_in_process_t_vars
Post-process, the stash vars are:
  x1: x1_defined_in_stash_set
  x2: x2_defined_in_stash_set
  x3: x3_defined_in_stash_set
Note that we could check more easily via _dump:
[Template::Stash] {
        x3               => x3_defined_in_stash_set
        x2               => x2_defined_in_stash_set
        global           => {
        }
        inc              => CODE(0x18503f8)
        dec              => CODE(0x1850458)
        x1               => x1_defined_in_stash_set
        _PARENT          => <undef>
    }