use strict;
use warnings;

my $fmt;

print
   map
   {
      sprintf qq{Date: %-12sFormat: %-s\n},
         $_->[0],
         $_->[1]
   }
   map
   {
       m
       {(?x)
          ^
          (?(?=\d\d(\D)\d\d(\D)\d{4}$)
             (?{$fmt = qq{DD${1}MM${2}YYYY}})
             |
             (?(?=\d{4}(\D)\d\d(\D)\d\d$)
                (?{$fmt = qq{YYYY${3}MM${4}DD}})
                |
                (?(?=\d\d(\D)\d\d(\D)\d\d$)
                   (?{$fmt = qq{DD${5}MM${6}RR}})
                   |
                   (?{$fmt = q{Date not valid}})
                )
             )
          )
       };
       [$_, $fmt];
   }
   map {chomp; $_;}
   <DATA>;

__END__
99/99/9999
99/99/99
99-99-9999
99-99-99
99-99
99-99-999
99:99:9999
99:99:99
99+99+9999
9999-99-99
9999:99:99
9/99/99
9999+99+99
99-99.9999
99.99-9999