One should not attempt to mix buffered I/O (like "read" or <FH>) with "select"

Most definitely. Data could be waiting in the buffer and select wouldn't know anything about it.

while (sysread(FIFO, $l, 255)) { $fifodata .= $l; }

Using while there defies the purpose of using select. If there's less than 256 bytes available, sysread will block the second time through the loop. You want: 

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

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(Big is good. It's not a problem if fewer bytes are available.)

The "ready for reading" half of your select loop should look something like: 

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...
}

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For fixed length records, the bottom loop might look like 

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

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For variable length records, s/// is convenient 

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

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In reply to Re: Malfunctioning select() call on a FIFO by ikegami
in thread Malfunctioning select() call on a FIFO by Llew_Llaw_Gyffes

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