Extension of that idea, ditching the conditionals (implicitly burying them in the regex's):

while(<DATA>){
  s/9999/YYYY/;                # 4-char year first
  s/(?<=[^9])99(?=[^9])/MM/;   # month is always in the middle
  s/(?<!YYYY.{4})99$/RR/;      # 2-char year is always at end, and won
+'t have a 4-char year already
  s/99/DD/;                    # the remaining 2-chars will be the day
  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
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