open IN, '<test1000000.dat' or die $!;
$offsets = pack 'V', 0; 
$offsets .= pack 'V', tell IN while <IN>;

print length $offsets;
4000008

sub readline_n{ 
    my( $fh, $line) = @_; 
    seek $fh, unpack( 'V', substr( $offsets, --$line*4, 4 )), 0; 
    scalar <$fh> 
}

print readline_n( \*IN, 500000 );
500000