[plt-scheme] running mzscheme in 3.99.0.23

I'm trying to figure out how things will change for students when we 
move to v400. When I write a program in Beginning Student like

(define (mysqr x) (* x x))
(mysqr 6)

and save it in "test.ss", then run "mzscheme test.ss" on the command 
line, nothing is printed out. But if I change the language to Module 
(mostly so the metadata goes away), put

#lang scheme/base

at the top of the file, save it as "test2.ss", and run "mzscheme 
test2.ss", 36 is printed out. I don't think I would have predicted that, 
but I'm sure there's a consistent explanation. What governs what is 
printed out, and can I change either of these behaviours? Thanks. --PR