Sum of elements below main diagonal of matrix

sub lower-triangle-sum (@matrix) { sum flat (1..@matrix).map( { @matrix[^$_]»[^($_-1)] } )»[*-1] }

say lower-triangle-sum 
[
    [  1,  3,  7,  8, 10 ],
    [  2,  4, 16, 14,  4 ],
    [  3,  1,  9, 18, 11 ],
    [ 12, 14, 17, 18, 20 ],
    [  7,  1,  3,  9,  5 ]
];

Output:

69