Challenge 1: A Subroutine with a Default — Possible Solution
====================================================================

use strict;
use warnings;
use feature 'say';

sub power {
    my ($base, $exponent) = @_;
    $exponent //= 2;
    return $base ** $exponent;
}

say power(5);        # 25
say power(2, 8);      # 256

Output:
25
256

WHY THIS WORKS AS AN ANSWER
------------------------------
my ($base, $exponent) = @_; reuses the chapter's own standard argument-
unpacking idiom exactly — both potential arguments are captured into
named scalars in one line, regardless of whether the caller actually
supplied the second one.

$exponent //= 2; reuses the chapter's own defined-or-assign operator
directly, from the greet_user example — when power(5) is called with
only one argument, $exponent is never set by @_ at all, so it starts
as undef, and //= correctly supplies the default of 2 in that case.
When power(2, 8) supplies both arguments, $exponent is already
defined as 8, so //= leaves it untouched.

return $base ** $exponent; reuses the exponentiation operator ** from
Course 1's operator coverage, applied to whichever values $base and
$exponent ended up holding — power(5) computes 5 ** 2 = 25 using the
default, and power(2, 8) computes 2 ** 8 = 256 using the explicitly
passed exponent.