Presumably this has something to do with writing the value back to an alias?
Good idea. I was testing an operation which doesn't change the variable, m/\s+/, which results in alias only about 2% faster than ref, independent from length.

One thing I liked to test was the influence the change of the string length by the operation has. When I changed s/\s+// to s/\s+//g the difference is again about 2% in favor of alias, otherwise it's about the same for s/\s/_/g which doesn't change the length.