Excerpt from HOP
sub tokens {
  my $target = shift;
  return sub {
    TOKEN: {
      return ['INTEGER', $1] if $target =˜ /\G (\d+) /gcx;
      return ['PRINT'] if $target =˜ /\G print \b /gcx;
      return ['IDENTIFIER', $1] if $target =˜ /\G ([A-Za-z_]\w*) /gcx;
      return ['OPERATOR', $1] if $target =˜ /\G (\*\*) /gcx;
      return ['OPERATOR', $1] if $target =˜ /\G ([-+*\/=()])/gcx;
      return ['TERMINATOR', $1] if $target =˜ /\G (; \n* | \n+) /gcx;
      redo TOKEN if $target =˜ /\G \s+ /gcx;
      return ['UNKNOWN', $1] if $target =˜ /\G (.) /gcx;
      return;
    }
  };
}
-----------------------------------------------------
cipher text 1        ciphertxt2          ciphertxt3
       |                |                     |
       +-------------+  +----------+          +----
       |             |  |          |          |
key--decryption      | decrypt     |      decrypt
       |             |  |          |          |
-------X             +--X          +----------X
       |                |                     | 
   plaintext 1       plntxt2               plntxt3
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If you modify cipher text 1 it is obvious that plaintext 1 will change. Plaintext 2 will also change because of the XOR with ciphertext 1 (following the decryption). Ciphertext 2 is not different though (nor is ciphertext3) so plaintext 3 will not be different.