This is what I came up with:

printf "%04s-%02s-%02s\n", map { @$a ? $_ + shift @$a : $_} (localtime
+,$a=[1900,1])[5,4,3];
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I'll break it into components to make it easier to digest.

(localtime,$a=[1900,1])
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Create a list that contains the date tokens provided by localtime, and also contains the newly initialized $a. $a does not need to be declared, as $a and $b are exempted from strict due to their use in sort.

(localtime,$a=[1900,1])[5,4,3]
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Take the above generated list, and extract indexes 5,4,3 (year,month,mday).

map { 
   @$a 
     ? $_ + shift @$a 
     : $_
} (localtime,$a=[1900,1])[5,4,3];
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Iterate over the list that was generated in the previous step. For each entry check if there are any values left in @$a. If yes, add the current value to the next value in @$a, and shift that item from the list.

In effect $a is a list of modifiers.