[plt-scheme] read-sexp?

Previously I was building libraries (and still am).
Now I have a need to replace a perl program. I want to
write this in scheme. There was a recent discussion on
using (write) and (read) for processing flat-files.
I have tested the (read-line), but can't guarentee(?)
what is written to the file will have sexps on a single
line.

I've thought about implementing a custom reader to
read the sexps. Has anyone done this? Is there some
code I can follow/learn from?

Thanks.

Mike