Pointer Artithmetic on DB_File values: i need it (or a very efficient solution for this problem) - Perl

Hope that some perl guru will help me:

I've implemented an inverted index using PERL + DB_File (using hash,
and b-tree). Therefore, i have my indexer and my searcher (a little
search engine, indead).

Given a word in the lexicon i stored in B-tree the corrispondent
inverted list,
using gap coding. For istance i have:

word_r -> id_1r:freq_1r,id_2r:freq_2r,id_3r:freq_3r,........,
id_nr:freq_nr
word_s -> ...

where freqij is just the freq of word_j in document doc_i.

gap encoding means that id_ij is expressed as difference with
id_(i-1)j

Now, i need to write some Cursor obj with state (call it
postingCursor).
The posting in fetched in a scalar and (a reference to it!) is saved
in the Cursor.

I need to write efficiently a function:

$postingCursorOBJ->getNext($minID)

which should return the next available (if any) id_lr >= minID.

1) I need some way to fetch from the disk, just the portion of
inverted list involved in current operations. Do tie or DBI support
this?

2) I need a way to unroll incrementally the inverted list ? Is this a
case
where pointer arithmethic could help ? or there is any efficent
solution
which i missing. I think that splitting it is the wrong way to do it.

PS: the original problem is to find the common IDs, given two inverted
lists
L1 and L2 which store IDs sorted in ascending way. This should be done
in
O(min {n, m)) instead of O(m+n).