merlyn's code uses the feature of list assignment in scalar context. This returns the number of elements being returned, which, in this case, is either 0 or 2. If the regex matches, then it returns $1 and $2, which means that $base and $ext get their values.

If the regex matches, then the two variables have values -- one can not be defined if the other isn't. Thus, merlyn's code does the right thing.

japhy -- Perl and Regex Hacker