For the sake of completeness, an ultra flexible solution:

use Time::Local;
use POSIX qw(strftime);

my ($Y, $M, $D) = unpack "A4 A2 A2", $date;
$M--;

my $newdate = strftime('%Y-%m-%d', gmtime timegm(0, 0, 0, $D, $M, $Y))
+;
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Of course for just changing the naming scheme on a few files or something like that, it does a ridiculous amount of work and I'd stick with a join.