Perl::Critic, by default, attempts to enforce the content of Perl Best Practices. PBP is intended as a set of guidelines and suggestions to make you think about how you write your code and whether it can be improved upon, not as a set of ironclad commandments.

Therefore, Perl::Critic's warnings should also not be taken as strict rules that you must comply with.

Perl::Critic gives you an explanation of why it objects to return undef;. Use that explanation to determine whether it is appropriate for you to return undef; in this case or not.

Also note that the Perl::Critic explanation says, "The solution is to just use a bare `return' statement whenever you want to return failure." If your intent is to return an undefined value rather than to indicate failure, then there's no problem with your return undef;, is there? (In that case, I would advise adding a comment stating that this is the intent so that a future maintainer who subscribes to PBP doesn't break your code in an attempt to "fix" it. Mentioning in the sub's documentation that a return value of undef is not intended to indicate failure would also be wise.)