You were almost there. Why did you remove the reordering of the date parts? 
$_ = sprintf '%04d-%02d-%02d', (split m:/:)[2, 0, 1] for @fields[10, 1
+4, 24, 26];

[download]

Update: Tested with 
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my @fields = 'a' .. 'z';
@fields[ 10, 14, 24, 26] = ('1/2/2016') x 4;
$_ = sprintf '%04d-%02d-%02d', (split m:/:)[ 2, 0, 1 ] for @fields[ 10
+, 14, 24, 26 ];
say join ',', @fields;

[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re: Formatting dates while parsing CSV by choroba
in thread Add leading zeros to days/months in dates while parsing CSV by BigRedEO

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