use POSIX qw/EAGAIN EFBIG ENOSPC/;

# stuff

unless (defined(syswrite($socket,$buffer))) {
    # Oops, didn't write.  How come?
    if ($! == EAGAIN) {
        # Nonblocking and socket not yet ready...
    } elsif ($! == EFBIG) {
        # Writing to a file that is too big for our filesystem.
    } elsif ($! == ENOSPC) {
        # Ooops, writing to a file and the disk is full.
    } else {
        # One of the other dozen or so reasons a write can fail.
    }
}

##</code><code>##

unless (defined(syswrite($socket,$buffer))) {
    # Oh dear, couldn't write.  Could it be that I'm
    # no longer connected?
    if (defined(getpeername($socket))) {
        # Yup, they're still there.  Must be a different error.
    } else {
        # Oops, the other side went away.
    }
}