To add onto @pturmel’s response, please look at the available scopes for the functions you’re attempting to use; saveFile() is only available in Vision Client scripting - not Perspective Sessions.
The writeFile() function IS available in Perspective, but ALL functions in Perspective run on the Gateway (so that’s where the file is written to), so you will never see a file on your local machine as a result of using writeFile().
There’s no mechanism in Perspective as of today that allows for exporting or downloading files of any type - short of coupling with the WebDev Module.