10 - Memoize

Trades space for time by caching return values 
If you call a sub again with the same arguments, perl uses the cached result
An extreme example:

# Compute Fibonacci numbers
sub fib {
    my $n = shift;
    return $n if $n < 2;
    fib($n-1) + fib($n-2);
}

use Memoize;
memoize('fib');