I was aware that tr did not interpolate, but the (very old) edition of Camel book said I could achieve the effect I wanted with eval.
The example they gave omitted a few key characters, and a more complete statement. I missed the necessity of having the backslash escape before the source string and also having ; characters to terminate both the expression to be eval'ed and also the eval statement itself.
Thanks to the various posts here I am now more enlightened.
Appreciated your use of Test:: in your post. I need to make better use of that package.