Thanks, I had already similar Tie::Array code but was struggling with the antics of the perltie implementation.

But TIMTOWTDI ...

I had the idea that blessing the variables into a package which overload stringification '""' should be even easier and more precise, because only placeholders inside strings would be caught.

Unfortunately I couldn't make this work without having leaking effects.° :(

Good news is, that at least the tie approach works with pads.

update

°) found the bug right after posting this :)