Create your own text control codes

This, in general would be a bad idea. It isn't smart to add a lot of overhead to core functions, especially for so little gain. That being said, something being a bad idea never stopped us before.

printf already has so many directives, most of the good mnemonics are already taken. Add a "commas" directive as %y and an "invert" directive as %z.

Some languages already have a commas directive as that one is actually useful. I doubt if any language has an "invert" directive.

This is really basic and sketchy. It only modifies printf, not sprintf, so probably isn't terribly useful as is... but it satisfies the task requirements. It actually does add new, non-standard directives to the core printf function, not just implement a separate formatting function to pre-format a string which is then passed to the printing function.

use Lingua::EN::Numbers;
use Acme::Text::UpsideDown;

sub printf (Str $format is copy, *@vars is copy) {
    my @directives = $format.comb(/ <?after <-[%]>|^> '%' <[ +0#-]>* <alpha>/);
    for ^@directives {
        if @directives[$_] eq '%y' {
            $format.=subst('%y', '%s');
            @vars[$_].=&comma;
        } elsif @directives[$_] eq '%z' {
            $format.=subst('%z', '%s');
            @vars[$_].=&upsidedown;
        }
    }
    &CORE::printf($format, @vars)
}

printf "Integer %d with commas: %y\nSpelled out: %s\nInverted: %z\n",
       12345, 12345, 12345.&cardinal, 12345.&cardinal;

Output:

Integer 12345 with commas: 12,345
Spelled out: twelve thousand, three hundred forty-five
Inverted: ǝʌᴉɟ-ʎʇɹoɟ pǝɹpunɥ ǝǝɹɥʇ ‘puɐsnoɥʇ ǝʌꞁǝʍʇ