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>

21-restful-apis-and-web-services.md

@@ -357,7 +357,7 @@ Now let's build a comprehensive API client that demonstrates best practices:
 
 ```perl
 #!/usr/bin/env perl
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 use feature 'signatures';
 no warnings 'experimental::signatures';
 
@@ -833,7 +833,7 @@ While REST dominates, GraphQL offers powerful query capabilities:
 
 ```perl
 #!/usr/bin/env perl
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 use feature 'signatures';
 no warnings 'experimental::signatures';
 
@@ -1145,7 +1145,7 @@ Comprehensive testing ensures reliable APIs:
 
 ```perl
 #!/usr/bin/env perl
-use Modern::Perl '2023';
+use Modern::Perl '2018';
 use Test::More;
 use Test::Mojo;
 use Test::Deep;