Note that the example program:

  (define (displayln obj)
    (display obj)
    (newline))
  
  (define cont #f)
  
  (displayln
    (call/cc 
      (lambda (k)
        (set! cont k)
        "cont set")))
  
  (begin
    (displayln "procedure called")
    (displayln "after procedure call")
    (cont "continuation called")
    (displayln "after continuation call"))

This is because every top-level form in the REPL has an implicit continuation "return to the REPL and read more input".

So after "continuation called" is printed, we go back to the prompt and await more input.

However, if this code is saved in a file and you run it (e.g. "guile my-script.scm") then the continuation of the top-level `displayln` call is the top-level `begin` form, and we enter an infinite loop.