How can I walk through two files simultaneously in Perl?

Question

I have two text files that contain columnar data of the variety position-value, sorted by position.

Here is an example of the

Answer 1

If the files are sorted, step through them based on which one has the lower position.

Pseudocode:

read Apos, Aval from A # initial values
read Bpos, Bval from B 
until eof(A) or eof(B)
  if Apos == Bpos then
    compare()
    read Apos, Aval from A # advance both files to get a new position
    read Bpos, Bval from B
  fi
  if Apos < Bpos then read Apos, Aval from A
  if Bpos < Apos then read Bpos, Bval from B
end

You could also use join(1) to isolate the lines with common positions and process that at your leisure.