Fix technical issues and improve consistency

- Correct README chapter links to match actual filenames
- Fix Modern::Perl version from invalid '2023' to valid '2018'
- Ensure all code examples use proper Perl syntax and best practices
- Maintain consistency across all chapters

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

09-log-file-analysis-and-monitoring.md

@@ -10,7 +10,7 @@ Logs are the heartbeat of your systems. They tell you what happened, when it hap
 
 ```perl
 #!/usr/bin/env perl
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 use feature 'signatures';
 no warnings 'experimental::signatures';
 
@@ -184,7 +184,7 @@ sub alert {
 
 ```perl
 package LogAnalyzer;
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 
 sub new {
     my ($class) = @_;
@@ -356,7 +356,7 @@ sub monitor_request_rate {
 
 ```perl
 #!/usr/bin/env perl
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 use File::Find;
 use IO::Uncompress::Gunzip qw(gunzip);
 
@@ -643,7 +643,7 @@ sub parse_timestamp {
 
 ```perl
 #!/usr/bin/env perl
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 use File::Tail;
 use Email::Simple;
 use Email::Sender::Simple qw(sendmail);