I would suggest you simply describe what is wrong with the practices you disagree with, or if they aren't wrong per se, why your alternative is better. That would go much farther than appealing to community standards.

In your first example, programmers in every language learn about the value of code re-use, so your senior developer knows he should do it; perhaps he is just unfamiliar with Perl modules. Surely those well-known advantages are case enough to make the change. And a senior developer should know from experience that tracking down and modifying each and every one of those duplicated code sections will take longer than it would to read a tutorial on how to use modules. If not longer in time, greater in annoyance! And a new skill would be gained.

The more general argument would apply for learning how to search CPAN. Or at least to learn about what functionality the core modules provide.