All Perl symbols that consist of punctuation only are automatically global. That means that you do not have to export _().
I think that this behavior should at least be documented. And maybe you can think of a better function name so that the global namespace does not get polluted.
If you are searching for another function name, please keep in mind that the double underscore __() is already exported by https://metacpan.org/pod/Locale::TextDomain and in relatively wide use.