I wanted to confirm if a variable delcared in a file is accessible and can be modified throughout the file, even inside a function, and that seems to be true.

my creates a variable that's visible in the current lexical scope (i.e. block or file).

Nothing like "masking earlier declaration of $val". It just treats it as if it were a different variable.

It is a different variable. And I'm also disappointed that this doesn't warn. The warning is only issued when you create two lexical variables in exactly the same scope.