When the value of a variable is undefined (as opposed to the variable being undefined), it means there's no way to predict what value the variable has.

In context, it means that $! could be true even no error occured. $! is only meaningful if sysread returned undef.

I see your point now. For some reason I thought that undef value would always evalutes to false in conditional statements in perl. However, when I thought more about it and given the fact that $! is a 'synonym' for C's errno, I see no guaranty that it would be reset to some specific value if no system error occurs and may indeed contain phantom residule value that could evaluate to true. Basically, without looking through perl's code there is no way to say which statement is true.

Here is a modified version of the read_msg sub...

sub read_msg {
    my ($sock, $bcount, $buf, $msg) = (shift, 0, '', '');
    my $bytes_read;   

    # Use sysread for stream-oriented sockets
    READ_MSG:
    while ($bytes_read = sysread($sock, $buf, 1024)) {
        # Handle partial read
        $bcount += $bytes_read;
        $msg    .= $buf;
    }

    # sysread returns undef on system error, check $!
    if (not defined $bytes_read) {

        # Just in case, save $! value
        my $error = $!;

        # To be safe here...
        redo READ_MSG if $error =~ /Resource temporarily unavailable/;

        # All other errors trigger unconditional return with error
        return ($msg, $bcount, $error);
    }

    # Return success
    return ($msg, $bcount, '');
}
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undef *is* always false. Again, I'm not talking about undef. That $!'s value is undefined (as opposed to $! being undefined) means $! could be 0, it could be -123, it could be the string "the answer to life, the universe, everything".