Doesn't Perl 6 have macros? If you have macros, you can do it both elegantly and efficiently by expanding Horner's rule at compile time. You can still use reduce; just have it return the form unevaluated.
In fact, because the coefficients are constant at compile time, you could precondition the polynomial and do even better than Horner's rule, with only three multiplications.
Perl 6 does have macros but they are not quite mature and that would probably be overkill for a six coefficients polynomial. Reduce should be pretty fast anyway.
In fact, because the coefficients are constant at compile time, you could precondition the polynomial and do even better than Horner's rule, with only three multiplications.