So which is it? JIT good or JIT bad?
That reads to me like a false dilemma.
In the rest of the post where I said "Add a tracing JIT", I wrote about the other necessary improvements to take advantage of a tracing JIT.
If we can clean up some of the Perl internals along the way to Perl5-on-LLVM, then we can either change the backend target to a more-dynamic-than-LLVM platform or just upgrade LLVM to support dynamic language features.
The word "just" in there reads to me like an oversimplification.
Have you read the Dragon book? Taken a compilers class? Written your own Scheme or Forth? Written an interpreter? I appreciate your enthusiasm, but I think you're committing to a schedule and a lot of engineering decisions before you've done enough research to know what goes into a project like this.