Looks like when the toplevel gets destroyed, the window{xAf} gets grinitialigzed to 0 under some circumstances ... probably a bug since as choroba shows stringification gives the correct value

This might be a bug worth reporting to the maintainer, rt://Tk

However, creating a Dialog instead of a toplevel naturally works as intended

sub roomSub {
  if (! Exists ($roomTl)) {  warn "textvariable"; 
#~     $roomTl = $mw->Toplevel();
    $roomTl = $mw->Dialog();
    $roomTl->Entry(
      -textvariable => \$window{xAf},
    )->pack(-fill => 'x');
  }
  $roomTl->Show();
}
[download]

Also, I won't bug you about tk scoping function argument passing