while (1) {
    my @random_int = sort {$b <=> $a} map {int(rand(100))} 0..1;

    my $correct = 0;

    last if !defined prompt(
        message     => "What is $random_int[0] - $random_int[1]? ('q' to exit) ",
        validate    => sub {
            my $raw = shift;
            return 0 unless looks_like_number($raw);
            return $correct = $raw == $random_int[0] - $random_int[1];
        },
        error => sub {
            my $raw = shift;
            return looks_like_number($raw)
                ? "Sorry, you are wrong. Have another go.\n"
                : "You need to enter a number.\n";
        },
        escape => qr/^q$/i,
    );

    print "Well done! You are correct!\n\n" if $correct;
}