Currency

No need for a special type in Raku, since the Rat type is used for normal fractions. (In order to achieve imprecision, you have to explicitly use scientific notation, or use the Num type, or calculate a result that requires a denominator in excess of 2 ** 64. (There's no limit on the numerator.))

my @check = q:to/END/.lines.map: { [.split(/\s+/)] };
    Hamburger   5.50    4000000000000000
    Milkshake   2.86    2
    END

my $tax-rate = 0.0765;

my $fmt = "%-10s %8s %18s %22s\n";

printf $fmt, <Item Price Quantity Extension>;

my $subtotal = [+] @check.map: -> [$item,$price,$quant] {
    my $extension = $price * $quant;
    printf $fmt, $item, $price, $quant, fix2($extension);
    $extension;
}

printf $fmt, '', '', '', '-----------------';
printf $fmt, '', '', 'Subtotal ', $subtotal;

my $tax = ($subtotal * $tax-rate).round(0.01);
printf $fmt, '', '', 'Tax ', $tax;

my $total = $subtotal + $tax;
printf $fmt, '', '', 'Total ', $total;

# make up for lack of a Rat fixed-point printf format
sub fix2($x) { ($x + 0.001).subst(/ <?after \.\d\d> .* $ /, '') }

Output:

Item          Price           Quantity              Extension
Hamburger      5.50   4000000000000000   22000000000000000.00
Milkshake      2.86                  2                   5.72
                                            -----------------
                             Subtotal    22000000000000005.72
                                  Tax     1683000000000000.44
                                Total    23683000000000006.16