I'm pretty sure you should check for a $SIG{PIPE}, but that signal will only be raised if you try to send data over a closed connection i.e. print to STDOUT under mod_cgi.

I'm not at all sure how this will interact with buffering; even if you've turned buffering off (set $|=1), apache might cache the script output before attempting to send it to the client. Maybe someone else here can fill you in on the details, otherwise, you'll have to experiment.