# the usual precautions
use strict;
use warnings;

# automatically die when open() or so fails
use autodie;

my $filename = shift @ARGV;
open my $handle, '<', $filename ;

# determine the block length
my $first_block = '';
$first_block .= <$handle> for 1..4;
my $block_length = length $first_block;

sub get_line {
   my ($handle, $block_length, $line) = @_;
   
   # assume the file starts with line 1:
   seek $handle, $block_length * ($line - 1), 0;
   my $result;
   my $read_len = read $handle, $result, $block_length;
   if ($read_len == $block_length) {
       return $result
   } else {
       die "Error while reading file: expected $block_length bytes, but got $read_len instead";
   }
}