The other advice is good, but I'll throw in a question:

Do you have to allow for uppercasing, like Yes and No? If so, you've got some additional work to do.

There are many ways to do it--my suggestion is to either lowercase the input with lc() before testing (if you're using eq) or to use the /i switch (if you're using the =~ operator). There are other ways, but I'd suggest these.