I've seen this before. What's happening, somehow, is that every time you write more than one line in a definition at the REPL in a Windows prompt, a capital R is being inserted at each newline. Arc compiles this to the Racket code _R, and when Racket executes this, it can't find the _R variable.
I seem to remember I used work around this by always pasting my multi-line definitions from a text editor rather than writing them directly at the REPL.
Oh, sorry. Now that I test it, I realize I remembered incorrectly.
The only time I get those spurious R characters is when I paste code into the REPL and then press enter manually. I don't get them when typing multi-line definitions directly at the REPL, and I don't get them if the code I'm pasting already has a line break at the end.
So the habit I've formed is to make sure the code I'm pasting already has a line break at the end.
I notice this issue also happens on Racket 5.3.3 -- I'm a few versions behind -- and it does not happen in the REPLs for Node.js or Clojure. It's some kind of bug in Racket. (Hmm... Racket's port.c has a bunch of spaghetti code for CRLF processing. Maybe the bug's in there somewhere.)