This is because it is in scalar context. See this example:

use Data::Dumper;

$string = 'A,B,C';

@test = ();
$test[0]{test} = split(/,/, $string);
print Dumper(\@test);

@test = ();
@{$test[0]{test}} = split(/,/, $string);
print Dumper(\@test);
__END__
$VAR1 = [
          {
            'test' => 3
          }
        ];
$VAR1 = [
          {
            'test' => [
                        'A',
                        'B',
                        'C'
                      ]
          }
        ];
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--
"I don't intend for this to take on a political tone. I'm just here for the drugs." --Nancy Reagan