From the source code, it looks like the CommPort internal lock _R_BUSY is never unlocked for some reason. Most likely, a "background" read is started, followed by another before the first one had a chance to finish (read_done), leading to this message.

This said, I know nothing about serial port handling in Perl, in particular on Win32, so you should probably contact the GPS module author as it looks like a bug in his code.