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use strict;
use warinings;
...
    $m->range(ndcoords($m(1:,1:)), 2)->reorder(2, 3, 0, 1)
        ->clump(2)->sumover;
}

sub submatrix_sum {
    my $w = shift;
...
    $m->range(ndcoords($m($w - 1:, $h - 1:), [$w, $h])->reorder(2, 3, 
+0, 1)
        ->clump(2)->sumover;
}