my $date = shift || '12/DEC/2001:17:23:09';

##</code><code>##

  use constant MONTH => {
    Jan => 1, Apr => 4, Jul => 7, Oct => 10,
    Fen => 2, May => 5, Aug => 8, Nov => 11,
    Mar => 3, Jun => 6, Sep => 9, Dec => 12,
  };

  ...

  my $mon = MONTH->{$mon};