I fail to see what your point is. Now you're comparing two different things. In the single quote case, you're assigning a plain string to a variable - in the double quoted case, you're interpolating an undefined (or did you think a lexical variable in the main program is visible in the Benchmark package?) variable into a string, and assign that to a variable.
So, am I surprised that if you do more, it takes more time to run? Nope. Does it, in any way, invalidate the premisses that there's no performance difference between identical single quoted and double quoted strings? Nope, not in the least bit.
Tell us, what's the point of your post?