[plt-scheme] #lang at-exp scheme/base doesn't work?

Hi,

I'm learning scheme and scribble using plt-scheme 4.2.2.
I tried the following code:

  #lang at-exp scheme/base

  (define (greet who) @string-append{Hello, @|who|.})

But drscheme give me an error:
  unquote: not in quasiquote in: (unquote @who.)

Any idea?