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use v5.12;
use warnings;
...
my @x= split',', q(a,a,a,a,b,c,c,a,a,d,e,e,e,e);

print Dumper slide { $a ne $b ? $b : () } "", @x;

$VAR1 = 'a';
$VAR2 = 'b';
...
$VAR4 = 'a';
$VAR5 = 'd';
$VAR6 = 'e';

$VAR1 = 'a';
$VAR2 = undef;
...
$VAR12 = undef;
$VAR13 = undef;
$VAR14 = undef;