Like @arr vs $#arr in Perl, I'm guessing @arr[0].elems was causing warnings for accessing the element just after the end of an array.

A possible alternative coding:

use v6;

my  @arr  = [
    [ 1.1,2.2,3.3,4.4,5.5 ],
    [ 10,20,30,40,50 ],
    [ 100,200,300,400,500 ],
    [ 1000,2000,3000,4000,5000 ],
];

dd @arr; # dump the matrix

for @arr {
      say ( .map: {.fmt: "%7.1f\t"} ).join
}

say "=======\t" x @arr[0].elems;
say ( (0..@arr[0].end).map: {@arr[*; $_].sum.fmt("%7.1f\t")} ).join;
#say ( ([Z] @arr).map: { .sum.fmt("%7.1f\t") } ).join;
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