List::Util::pairmap would be an existing solution very similar to the new feature. It also seems to run quite quickly (but I can't compare it to for_list since I don't have a new-enough perl).

use List::Util qw/ pairmap /;
sub a_pairmap {
    my $i;
    pairmap { $i += $b } @vals;
    return $i;
}
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