Simple logging

sub add-numbers(&log, $x, $y) {
    log("Adding $x and $y");
    $x + $y;
}

sub multiply-numbers(&log, $x, $y) {
    log("Multiplying $x and $y");
    $x * $y;
}

sub my-program(&log, $x, $y) {
    say add-numbers(&log, $x, $y);
    say multiply-numbers(&log, $x, $y);
}

sub MAIN() {
    my &*log;

    say "\n", 'No logging at all.';
    my-program(-> $msg { }, 1, 2);

    say "\n", 'Log only additions (filtering).';
    my-program({ .say if m/Adding/ }, 1, 2);

    say "\n", 'Log in all caps.';
    my-program({ .uc.say }, 1, 2);

    say "\n", 'Log all messages.';
    my-program(&say, 1, 2);
}