Neither File::Lib nor Lib::File (and you use both names so it's impossible to know which one you really mean) are on the CPAN so it's hard to be much help as we can't see the code of the module in question. Where did this module come from?

An export tag like :filetest is usually created using the Exporter module and the @EXPORT_TAGS variable. You might like to look at the documentation for Exporter to see how it's supposed to work and then compare that with how your module works.