A recursive solution:

sub c {
    my$a=pop;$a==1?1:8**($a-1)+c($a-1)
}
[download]

And a non-recursive one:

sub c {
    my$a=1;$a+=8**$_ for(1..pop-1);$a
}
[download]

updates on that last bit of code

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain