fibonacci in perl

fib.pl

#!/usr/bin/perl

# O(n) in time, O(1) in space
sub fib {
    my $n = $_[0];
    my $a = 0;
    my $b = 1;

    for ($i = 0; $i < $n; $i++) {
        ($a, $b) = ($a+$b, $a);
    }
    return $a;
}