Why use a conversion hash when you can just convert?

UPDATE: initally missed leading year format, updated code to handle it

use strict; use warnings;

while(<DATA>){
    s/^(\d{2,4})(\D)(\d{2})(\D)(\d{4}|\d{2})/
        (
            length $1 > 2 
            ? 'YYYY'
            : 'DD'
        )
        ."$2MM$4"
        .(
            length $5 > 2 
            ? length $1 <= 2
              ? 'YYYY' 
              : 'RR'
            : 'DD'
        )/ex;
    print;
}

__DATA__
99/99/9999
99/99/99
99-99-9999
99-99-99
99:99:9999
99:99:99
99+99+9999
9999-99-99
9999:99:99
9999+99+99
[download]

Output:

DD/MM/YYYY
DD/MM/DD
DD-MM-YYYY
DD-MM-DD
DD:MM:YYYY
DD:MM:DD
DD+MM+YYYY
YYYY-MM-DD
YYYY:MM:DD
YYYY+MM+DD
[download]