Challenge 2: Count Lines Matching a Pattern — Possible Solution
====================================================================

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

sub count_matching {
    my ($filename, $pattern) = @_;
    my $count = 0;

    open(my $fh, '<', $filename) or die "Can't open $filename: $!";
    while (my $line = <$fh>) {
        $count++ if $line =~ $pattern;
    }
    close($fh);

    return $count;
}

my $errors = count_matching('app.log', qr/ERROR/);
say $errors;

WHY THIS WORKS AS AN ANSWER
------------------------------
my ($filename, $pattern) = @_; reuses Chapter 7's own argument-
unpacking idiom, taking both the file to search and the pattern to
search for as separate parameters, rather than hardcoding either one.

open(my $fh, '<', $filename) or die "..."; reuses this chapter's own
3-argument open with error checking, applied to whichever filename
was passed in.

$count++ if $line =~ $pattern; combines two things directly from
earlier chapters: Chapter 5's postfix conditional style (the
increment only happens if the condition is true, in one line) and
Chapter 6's =~ binding operator — but here $pattern is a variable
holding a precompiled regex (built with qr/ERROR/, Perl's "quote
regex" operator for storing a pattern in a scalar) rather than a
literal /pattern/ written directly in the code, which is what makes
the subroutine reusable for ANY pattern the caller wants to search
for, not just one hardcoded into count_matching itself.

return $count; reuses ordinary explicit return (Chapter 7) to hand
back the running total once every line has been read — the same
accumulator-variable pattern used throughout this course's earlier
loop examples, just counting regex matches instead of something else.