The general variable assignment procedure in Scheme is set!, as in (set! x 3). But this is an error when x is undefined.
I just noticed compile-allow-set!-undefined the other day:
Welcome to MzScheme v4.2.1 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
> (set! a 3)
set!: cannot set undefined identifier: a
> (compile-allow-set!-undefined #t)
> (set! a 3)
>
