Actually, [@{$d[$mapkey]}] and $d[$mapkey] are not the same. The first makes a copy, the second doesn't.

#!/usr/bin/perl

use strict;
use warnings;  

my @d      = ([0, 1, 2], [3, 4, 5], [6, 7, 8]);
my $mapkey = 1;
    
sub show {
    local($") = ", ";
    print("[", join(", ", map({"[@$_]"} @d)), "]\n");
}

my $one    = [@{$d[$mapkey]}];
my $two    = $d[$mapkey];

                   show;  # Show array. Unmodified.
$one->[1] = "foo"; show;  # Array is still unmodified.
$two->[1] = "foo"; show;  # This modifies the array.
 
__END__
[[0, 1, 2], [3, 4, 5], [6, 7, 8]]
[[0, 1, 2], [3, 4, 5], [6, 7, 8]]
[[0, 1, 2], [3, foo, 5], [6, 7, 8]]
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