You highlighted important things there, big thanks!

I agree, that using @_ in args is more straightforward, and it should be used instead of shift.
Also, it's a good idea to leave comments for private methods.

...is there a reason you're intentionally returning undef?

As I know, perl returns the last value from a block, so returning undef is a way to protect from random bugs. Is it unnecessary or there is a better way?