The fibnonacci sequence implemented as a method on an autoboxed integer/scalar class, with caching, for demonstration purposes.

gistfile1.pl

#!/usr/bin/perl

# This turns on function signatures and autoboxing
# amongst a lot of other things.
use perl5i::2;

# This class adds methods callable on all scalar variables.
{
    package SCALAR;
    use perl5i::2;

    method fibonacci {
        # This cache is attached to the fibnoacci method.
        # It is initialized only once.
        state $cache = {
            0   => 0,
            1   => 1
        };

        return $cache->{$self} //= ($self-1)->fibonacci + ($self-2)->fibonacci;
    }
}

say 20->fibonacci;