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);
    }
}