It's probably something in that part of the code which you haven't shown us (or in your data). This works fine for me:

#!/usr/bin/env perl 
use strict;
use warnings;
use Test::More tests => 1;

my @have = ('1/2/2003', '4/5/2006', '10/11/2012');
my @want = ('2003-01-02', '2006-04-05', '2012-10-11');
$_ = sprintf '%04d-%02d-%02d', (split m:/:)[2,0,1] for @have;
is_deeply (\@have, \@want);
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