passing unused data to syswrite() seems like an odd thing to do.

Since you can't control how many bytes actually get written, it makes it very easy to use syswrite: 

my $to_write = length($msg);
while ($to_write) {
    my $written = syswrite($fh, $msg, $to_write, -$to_write)
        or die "syswrite: $!\n";
    $to_write -= $written;
}

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In reply to Re^3: Random read/write on a block device by ikegami
in thread Random read/write on a block device by FloydATC

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