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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";
...
        $offset = tell($data_file);
    }
}

# usage: line_with_index(*DATA_HANDLE, *INDEX_HANDLE, $LINE_NUMBER)
# returns line or undef if LINE_NUMBER was out of range
...
    seek($data_file, $d_offset, 0);
    return scalar(<$data_file>);
}