Update p5-Object-Event to 1.23.

Changes from previous:
1.23    Thu Aug  4 10:52:32 CEST 2011
        - added small safety-check to unreg_cb.

1.22    Thu Mar 10 17:17:58 CET 2011
        - fixed a bug where event methods were not registered correctly.
        - found a bug with using the guard to track cb registrations.

(hiramatsu)