Challenge 2: Count and Print with -ne — Possible Solution
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perl -ne '$c++ if /WARN/; END { print "$c\n" }' app.log

WHY THIS WORKS AS AN ANSWER
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-ne wraps the code in this chapter's own implicit while (<>) loop,
running once per line of app.log with no automatic printing (that's
what distinguishes -n from -p) — appropriate here since the goal is a
single final count, not a transformed copy of every line.

$c++ if /WARN/ reuses a bare regex match against $_ (the loop's
default variable, holding the current line) — Course 1, Chapter 6's
own pattern-matching-with-no-explicit-variable idiom — incrementing
$c only on lines that actually contain "WARN".

END { print "$c\n" } is this chapter's own END block, which runs
exactly once after every line has been processed rather than once per
line — the correct place to print a final summary value rather than
printing it (and getting it wrong) inside the per-line loop itself.

If app.log contains no lines matching /WARN/ at all, $c is never
incremented and remains undef; printing "$c\n" in that case prints an
empty line (with a "use of uninitialized value" warning under
use warnings) rather than 0 — worth being aware of, though not
relevant for a file that does contain matches.