Challenge 1: Replace Text with -pe — Possible Solution
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Preview only (no changes written to the file):

perl -pe 's/cat/dog/gi;' pets.txt

Apply the change in place, keeping a backup at pets.txt.bak:

perl -i.bak -pe 's/cat/dog/gi;' pets.txt

WHY THIS WORKS AS AN ANSWER
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s/cat/dog/gi reuses Course 1, Chapter 6's own s/// substitution operator
directly — /g makes it replace every occurrence per line rather than
just the first, and /i makes it case-insensitive, so "Cat", "CAT", and
"cat" are all matched.

Running the command WITHOUT -i first (the preview version) prints the
transformed output straight to the terminal without touching
pets.txt at all — exactly the safety check this chapter's own
warn-box recommends before ever running an in-place edit for real.

Adding -i.bak switches to actually editing the file, but the .bak
suffix means Perl first copies the original, untouched file to
pets.txt.bak before overwriting pets.txt — so if the substitution
turns out to be wrong, the original is still recoverable, unlike a
bare -i with no suffix at all.