This error message is returned when the client breaks the connection while your script is trying to write to the client. With Apache 1.3.x, you should see the rwrite messages only if LogLevel is set to debug. (Prior to mod_perl 1.19_01, there was a bug that reported this debug message regardless of the value of the LogLevel directive.)

Generally LogLevel is either debug or info. debug logs everything, and info is the next level, which doesn't include debug messages. You shouldn't use debug mode on a production server. At the moment there is no way to prevent users from aborting connections.