Firstly, good for you! Code contributions such as this are what keeps the whole open-source ecosystem running.

I have done that 'to do' and tested it and it works.

Also very good. When you say you "tested it" do you mean you wrote a test script? That would be ideal - module authors love code contributions which come along with their own test script too.

There's no Pull Request system like with a Git repo?

No, there is not. What there is is https://rt.cpan.org/ where you can submit your patch and test script so that not only does the module author get to see it but all the co-maintainers (if there are any) will also be alerted to it. If the author (or principal maintainer if different) is indisposed it means that your contribution will still receive attention. It also means that anyone else who spots the "to do" section could find your contribution there even if it has not yet been merged into the distribution.

Some modules are additionally on other distributed code platforms such as GitHub, GitLab, BitBucket, etc. and if the module is one of those you could also go that route with your contribution.

If you would care to say which module you are talking about then we would be better placed to provide specific advice.