Just some ideas on how you can answer your questions:

For representing infinite lists I recommend you to read Mark Jason Dominus' excellent book named Higher Order Perl, especially the chapter on infinite lists (where he is using closures for iterators and promise-forcing streams). It is available online in pdf format: http://hop.perl.plover.com.

And for your updated question: given the finite parts (a1, ..., aN), (b1, ..., bM), if you take min(a1+bM, b1+aN), you are safe to print any sum less than or equal to this threshold as a beginning sequence of the final list. Anything above this threshold cannot be considered final.

update: First I've written this: min(a1, b1) + min(aN, bM), but this is wrong, corrected above. And minor grammatical edits, arrgh...