perldoc Date::Manip will answer your question.

The short version is:

                                               ... All parts of
         the date can be separated by valid separators (space,
         "/", or ".").  The separator "-" may be used as long as
         it doesn't conflict with an ISO 8601 format, but this is
         discouraged since it is easy to overlook conflicts.  For
         example, the format MM/DD/YY is just fine, but MM-DD-YY
         does not work since it conflicts with YY-MM-DD.  To be
         safe, if "-" is used as a separator in a non-ISO format,
         they should be turned into "/" before calling the
         Date::Manip routines.  As with ISO 8601 formats, all
         separators are optional except for those given as a "/"
         in the list above.