my $i = 78187493530;

write_index($i);
print read_index();  # 78187493530

sub write_index {
    my $i = shift;
    open my $f, ">", "myindex" or die $!;
    my $pi = pack("CN", $i / 2**32, $i % 2**32);
    print $f $pi;    # writes 5 bytes
    close $f;
}

sub read_index {
    open my $f, "<", "myindex" or die $!;
    read $f, my $pi, 5;
    my ($C, $N) = unpack "CN", $pi;
    return $C * 2**32 + $N;
}