Or

use strict;
use warnings;

my %a = (one => [[],[],[]]);
print $#{$a{"one"}} + 1;
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Indeed what is your @a{one}?

use Data::Dumper;

use strict;
use warnings;

my %a = (one => [ ['a','b'],['c','d'] ]);
print Dumper(@a{one});
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output:

$VAR1 = [
          [
            'a',
            'b'
          ],
          [
            'c',
            'd'
          ]
        ];
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