my $rv = sysread(FIFO, $fifodata, 64*1024, length($fifodata));

if (!defined($rv)) {
    ... handle error ...
}

if (!$rv) {
    ... handle eof. There might be a partial message in $fifodata ...
}

while (...$fifodata contains a complete message...) {
    ...extract message from $fifodata...
    ...process message...
}