side effects

Something extra that happens when you evaluate an "expression". Nowadays it can refer to almost anything. For example, evaluating a simple assignment statement typically has the "side effect" of assigning a value to a variable. (And you thought assigning the value was your primary intent in the first place!) Likewise, assigning a value to the special variable 0 ($AUTOFLUSH) has the side effect of forcing a flush after every write or print on the currently selected filehandle.