No need for recursion, here's my obfuscation:

sub permutation {
    my ($prob, $start) = (1, shift);
    $prob *= $start-- foreach (1 .. $_[0]);
    $prob;
}
[download]

Another approach that requires thinking a bit backwards:

sub permutation {
    my $prob = 1;
    $prob *= $_ foreach ((($_[0]+1) - $_[1]) .. $_[0]);
    return $prob;
}
[download]

Even shorter and harder on the eyes:

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

I stand by the first I posted, though. Anyone who can get this down to a single statement gets my respect.