I believe what happens is that strict does a compile-time check to make sure the typeglob slot for that name and that type in the current package contains a valid reference (after checking that there's no in-scope my variable of that name). Here's a snippet from Exporter/Heavy.pm in Perl 5.8.6 that seems to do the meat of the work:

*{"${callpkg}::$sym"} =
            $type eq '&' ? \&{"${pkg}::$sym"} :
            $type eq '$' ? \${"${pkg}::$sym"} :
            $type eq '@' ? \@{"${pkg}::$sym"} :
            $type eq '%' ? \%{"${pkg}::$sym"} :
            $type eq '*' ?  *{"${pkg}::$sym"} :
            do { require Carp; Carp::croak("Can't export symbol: $type
+$sym") };
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Shouldn't require XS AFAICT.