The my keyword makes the variable into a private "lexical" variable. Code in other packages cannot refer to it by name.

The our keyword works similarly, but turns the name into a package variable. Then you can refer to it by its package. For example, if declared inside package MyPackageName, then other packages can refer to it as $MyPackageName::res.

I could just tell you that files included with require don't need package names and all of that, but that path leads to madness when your project grows.