Fully cognizant that I'm rolling the dice by responding to this comment, but isn't picking a variable syntax that could resolve in the unsafe way a "you're holding it wrong" waiting to happen?

  % set ex1 "SELECT * FROM FOO WHERE alpha=$bravo"
  can't read "bravo": no such variable
  % set ex2 "SELECT * FROM FOO WHERE alpha=?1"
  SELECT * FROM FOO WHERE alpha=?1

It's an idiomatic Tcl thing. E.g. `expr` (the standard word used to evaluate infix expressions like `1+2`) does the same exact thing, handling the expansion itself and expecting the caller to use {} to ensure that a variable expansion not incorrectly treated as a bunch of operators. Similarly, when you're writing the condition for a loop, you need to use {} to delay expansion so that the loop word can do it anew for each iteration. One can argue that this is somewhat error prone, but at least there's a consistent pattern here, and once you know what it is and why it exists, it's pretty straightforward.