I would probably read the whole file once and store the data in a hash whose key would be the concatenation of the two IPs (or it could also be a hash of hashes). Then,, I would read the file once more and for each "$timestamp $ip1 $arrow $ip2 $length " line loopup for $ip2/$ip1 in the hash. That's the basic idea, I can't really go into more details, because you did not give enough details (such as: can there be several lines with the same ip1/ip2 combination? this would impact how to exactly store the data in the hash).
in reply to Parsing my script out put?
This could actually be done in only one go, but the mechanics is a bit more complicated.