This duplicates my understanding of your code without the temporary hash.

use strict;
use warnings;
use List::Util qw(uniq);
use Data::Dumper;
my $group = q(ABC1);
my $value = q(xyz);
my $href = {
    values=> [qw(abc xyz abc)],
};

push @{$href->{test}}, {group=>$group};

$href->{test}[-1]{values} =
    ( defined $href->{"values"} ) ?
        [sort(uniq($value,@{$href->{"values"}}))] :
        $value
        ;

print Dumper $href;
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