my $buf;
while (1) {
  my $nr = sysread(IN, $buf, 1024, length($buf));
  unless ($nr) { ...handle EOF or error... };
  ...
  if (length($buf)) {
    my $nw = syswrite(OUT, $buf, length($buf));
    unless (defined($nw)) { ...handle error... }
    substr($buf, 0, $nw, '');
  }
  ...
}