my $req = Apache::FakeRequest( uri='/foo/bar', is_main=>1...);

*Apache::FakeRequest::request_uri = sub{ 
   #gen the uri here 
};
[download]

This should generate a warning

Subroutine Apache::FakeRequest::request_uri redefined at...
[download]

Which is your indication that it worked and only affects the FakeRequests not the real ones.