Cool, but I couldn't resist rakufying your "deflate" subroutine:

    sub deflate($power is copy, $rate, $years) {
        $power *= 1 + $rate for ^$years;
        $power * 100
    }
    sub MAIN(:$rate = 0.0225, :$years = 10)
    {
        print qq:to/EOF/;
        After $years years of deflation at a rate of {$rate * 100}% per year,
        purchasing power is &deflate(1,$rate,$years)% of what it was initially.
        EOF
    }

Which I think reads quite a bit better :-)