[chromatic]

Perl was a great language back at the 90's and early 2000 - it doesn't now.

Professional Perl programmers understood this coding error as a coding error at least in 2000, in my personal experience. If you squint, you can see it as a poorly designed interface in the CGI module (though I'm not sure how you would fix it), but your examples are passing untrusted, unvetted user input to sensitive code.

Professional programmers have understood that as bad practice for multiple decades.

Did all of those programmers and maintainers never read the tutorial for the language?

I'm certain if you went back in time and asked "What happens if a query string contains multiple parameters of the same name?" many people would look very confused, as if they'd never considered such a thing were possible. In other words, the answer to your question is "No, most web programmers neither read nor understood the documentation, because the state of web programming in those days was terrible."