Suppose I struggle to see at night, but was witness to a crime that took place and have been asked to give testimony. At the stand, I am asked whether I saw the defendant committing the crime, and although I am rather confident I did, I am not sure because the crime took place at night. However, instead of simply saying that, I say 'yes'.
Assuming no one asks me at any point whether I have poor eyesight, have I committed perjury by not explicitly saying that I am not sure?
I admit that this is a bit of a silly example, but it's the first thing that came to mind and sort of demonstrates the point.
So to restate the question:
Is answering a question with a yes or no perjury if you are not sure of your answer? Would that be considered not telling "the whole truth"?