The filepicker is not exported by default, it is part of the :widgets export tag.
You can either use use XUL::Gui ':all'; to get everything, or use use XUL::Gui qw(:default filepicker); to get the default set of imports and the filepicker.
Take a look at the EXPORT heading for more details.
Sorry the documentation is horrible, its a work in progress 🙂
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solved How does filepicker work in XUL::Gui?