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#!/usr/bin/env perl
use strict;
...
    return 1 if $num<3;
    return fibo($num-1) + fibo($num-2);
}

$ perl fibo.pl
fib(1)=1  (6e-06 sec)
...
fib(40)=102334155  (38.201329 sec)
fib(41)=165580141  (62.30559 sec)
^C

use Memoize;
memoize('fibo');

$ perl fibo.pl
fib(1)=1  (8e-06 sec)
...
fib(98)=1.35301852344707e+20  (6e-06 sec)
fib(99)=2.18922995834555e+20  (6e-06 sec)
fib(100)=3.54224848179262e+20  (6e-06 sec)