Because it's a simple statement like any other, not a special compiler directive compound statement that introduces a block. You need to put a semicolon after it whenever you'd need to put a semicolon after any simple statement. See perlsyn:

Simple Statements
  The only kind of simple statement is an expression evaluated for its
  side effects. Every simple statement must be terminated with a
  semicolon, unless it is the final statement in a block, in which cas
+e
  the semicolon is optional. (A semicolon is still encouraged if the b
+lock
  takes up more than one line, because you may eventually add another
  line.) Note that there are some operators like "eval {}" and "do {}"
  that look like compound statements, but aren't (they're just TERMs i
+n an
  expression), and thus need an explicit termination if used as the la
+st
  item in a statement.
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Update: Fixed up terminology as per replies, below