Hi Adam!
For string manipulation, the proper encoding has to be considered. It's better to use the provided [#:guess-encoding=#f] [#:encoding=#f] as well.
You may take a look at call-with-input-file in document:
https://www.gnu.org/software/guile/manual/html_node/File-Ports.html

BTW, in your case, you can ignore the binary mode.

Best regards.