Jensen's Device

Rather than playing tricks like Perl 5 does, the declarations of the formal parameters are quite straightforward in Raku:

sub sum($i is rw, $lo, $hi, &term) {
    my $temp = 0;
    loop ($i = $lo; $i <= $hi; $i++) {
        $temp += term;
    }
    return $temp;
}

my $i;
say sum $i, 1, 100, { 1 / $i };

Note that the C-style "for" loop is pronounced "loop" in Raku, and is the only loop statement that actually requires parens.