open(IN, $oneper) or die "Can't open file $oneper for reading: $!\n";
open(INDEX, "+>$file.idx") or die "Can't open $file.idx for read/write: $!\n";

build_index(*IN, *INDEX);

# usage: build_index(*DATA_HANDLE, *INDEX_HANDLE)
sub build_index {
    my $data_file  = shift;
    my $index_file = shift;
    my $offset     = 0;

    while (<$data_file>) {
        print $index_file pack("N", $offset);
        $offset = tell($data_file);
    }
}

##</code><code>##

# usage: line_with_index(*DATA_HANDLE, *INDEX_HANDLE, $LINE_NUMBER)
# returns line or undef if LINE_NUMBER was out of range
sub line_with_index {
    my $data_file   = shift;
    my $index_file  = shift;
    my $line_number = shift;

    my $size;               # size of an index entry
    my $i_offset;           # offset into the index of the entry
    my $entry;              # index entry
    my $d_offset;           # offset into the data file

    $size = length(pack("N", 0));
    $i_offset = $size * ($line_number);
    print "size is $size offset is $i_offset\n";
    seek($index_file, $i_offset, 0) or return;
    read($index_file, $entry, $size);
    $d_offset = unpack("N", $entry);
    seek($data_file, $d_offset, 0);
    return scalar(<$data_file>);
}