use List::Util qw( min );

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

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