I agree, recursive function follows better the mathematical definition and is easier to implement (no useless loop).

Here is an example (if anyone is interested as derpp solved):

sub fibonacci {
    my $n = shift;

    return undef if $n < 0;

    my $f;
    if ($n == 0) {
        $f = 0;
    } elsif ($n == 1) {
        $f = 1;
    } else {
        $f = fibonacci($n-1) + fibonacci($n-2);
    }
    
    return $f;
}

print fibonacci(25);
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