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

##</code><code>##

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...
}

##</code><code>##

while (length($fifodata) > $MSG_SIZE) {
    my $msg = substr($fifodata, 0, $MSG_SIZE, '');
    process_msg($msg);
}

##</code><code>##

while ($fifodata =~ s/^([^\n]*\n)//) {
    my $msg = $1;
    process_msg($msg);
}