Which was my point: The compiler looks at it differently. So there is a difference at compile time, as the compiler decides whether or not it needs to interpolate. I don't know how much time that takes. (If any: It is possible that both code-paths are equivalent in time, after all.)

In theory that could be a major hit at startup. I doubt it however. And it depends on the situation: In the current context we are talking about a script that is being used in some CGI environment, so if the hit was really bad it is likely the script could be run under mod_perl, which would remove it from consideration. (If it isn't already.)