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$arr[0] = @alpha;

$arr[0] = \@alpha;

$arr[0] = [ @alpha ];

foreach( \@alpha, \@beta ) {
    push @arr, $_;
}

foreach( [ @alpha ], [ @beta ] ) {
    push @arr, $_;
}

my $len = @arr;         # Number of lists contained in @arr.
my $len = @{ $arr[0] }; # Number of elements contained in the list
                        # referenced by $arr[0].