Like so much in life, it all depends on operator precedence. In the first case, your && binds more tightly to @REPORT than grep does. This means that as long as @REPORT is true in scalar context (i.e. is not an empty list), the && executes your do block. By adding parenthesis, you clarify the intended precedence and grep gets fed @REPORT and not the presumed return of your do block (which would never return because of your last).

Update: For the technical correctness, the && operator is higher precedence than the , operator (Comma Operator), which creates generates the list fed into grep. Thus swapping && to the low precedence and fixes the issue.