first try

multi MAIN(@ARGS where{ @ARGS.any == '--version' | '-V'}){
    say "$pkg $pkg_version";
    exit(0);
}

multi MAIN( Bool :$help,
            Str  :$export_dir! where
                     { .IO.d // die "$pkg: export directory does not exist." },
            Str  :$library_root where
                     { .IO.d // die "$pkg: export directory does not exist." }
                     = $export_dir,
            Str  :$dbfile where
                     { .IO.f && .IO.r // die "$pkg: banshee database does not "
                                             ~ " exist or is not readable" }
                     = "%*ENV<HOME>/.config/banshee-1/banshee.db"
        )
{
 
    exit 1;
}