seek($fh, $bytes_in, SEEK_SET)
   or die($!);

my $to_read = $bytes_out-$bytes_in;
while ($to_read > 0) {
   my $rv = read($fh, my $buffer, $chunk_size);
   die($!) if !defined($rv);
   die("Premature eof") if !$rv;
   substr($buffer, $to_read) = '' if $rv > $to_read;
   print($buffer);
   $to_read -= $bytes;
}