Challenge 3: Case-Insensitive Replace-All — Possible Solution
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use strict;
use warnings;
use feature 'say';

my $text = "Cat sat with the cat and a CAT";

$text =~ s/cat/dog/gi;

say $text;

Output:
dog sat with the dog and a dog

WHY THIS WORKS AS AN ANSWER
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$text =~ s/cat/dog/gi; reuses the chapter's own s/// substitution
form directly, modifying $text in place (no /r modifier used here,
since the challenge doesn't ask to preserve the original).

The /g flag reuses the chapter's own modifier table entry — without
it, s/// only replaces the FIRST match in the string, leaving "cat"
and "CAT" (the second and third occurrences) untouched. With /g, all
three occurrences ("Cat", "cat", "CAT") get replaced.

The /i flag, also reused directly from the chapter's own modifier
table, makes the match case-insensitive — without it, the pattern
cat would only match the lowercase "cat" in the middle of the
sentence, missing "Cat" (capitalized first letter) and "CAT" (fully
uppercase) entirely.

Combining both flags together (/gi) is what's required to replace
every occurrence regardless of case in a single substitution call —
each flag solves a different half of the problem (how MANY matches
to replace, and which CASE variations count as a match), and the
challenge specifically needs both at once.