sub equilibrium-index(@arr) {
  my ($left, $right) = (0, [+] @arr);
  my $ret;
  while @arr.elems -> $i {
    $right -= @arr[$i];
    $ret = $i if $left == $right;
    $left += @arr[$i];
  }
  return $ret ?? $ret !! -1;
}

sub MAIN()
{
    my @arr = (1,3,5,7,9);
    put equilibrium-index(@arr);
}

Use of uninitialized value of type Any in numeric context
  in block  at ch-2.raku line 6