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use strict;
use Benchmark;

...
    my $prob = 1;
    return (map { $prob *= $_ } (($_[0] - $_[1] + 1) .. $_[0]))[-1];
}

Benchmark: timing 1000000 iterations of p1, p2, p3, p4, p5, p6, p7...
        p1: 61 wallclock secs (45.42 usr +  0.16 sys = 45.58 CPU)
        p2: 45 wallclock secs (35.92 usr +  0.11 sys = 36.03 CPU)
...
        p5: 58 wallclock secs (46.11 usr +  0.14 sys = 46.25 CPU)
        p6: 55 wallclock secs (43.51 usr +  0.15 sys = 43.66 CPU)
        p7: 85 wallclock secs (68.11 usr +  0.22 sys = 68.33 CPU)