24 game

use MONKEY-SEE-NO-EVAL;

say "Here are your digits: ", 
constant @digits = (1..9).roll(4)».Str;

grammar Exp24 {
    token TOP { ^ <exp> $ { fail unless EVAL($/) == 24 } }
    rule exp { <term>+ % <op> }
    rule term { '(' <exp> ')' | <@digits> }
    token op { < + - * / > }
}

while my $exp = prompt "\n24? " {
    if try Exp24.parse: $exp {
        say "You win :)";
        last;
    } else {
        say (
            'Sorry.  Try again.' xx 20,
            'Try harder.' xx 5,
            'Nope.  Not even close.' xx 2,
            'Are you five or something?',
            'Come on, you can do better than that.'
        ).flat.pick
    }
}

The MONKEY-SEE-NO-EVAL pragma enables the dangerous EVAL function, which will compile and execute even user input. In this example, the grammar used to parse the input should ensure that only safe expressions are evaluated.