Never code for "efficiency". Instead code for clarity and maintainability. Compared with almost anything else your code does, declaring variables takes no time at all. Even if it took a huge amount of time like 1/1000 of a second, that is still tiny compared to the time it takes to read a line from disk and process it. And even if it represented a large portion of the time for each loop iteration, unless you are processing thousands of lines that overhead just isn't noticeable. In practice the overhead is likely to be much less than 1 millionth of a second and nothing to worry about ever.

Just remember: premature optimization is the root of all evil.