AFAIS

• it's only operating on UCS-2 and ignoring the planes above
• it's ignoring whitespaces by it's own definition of whitespace
• it's avoiding some control characters

So not just a simple rotate by 0x8000 = 2^15!

One needs

• an ordered list of all allowed characters.
• divide them by two
• map them in a lookup hash with 2^16 entries
• the ignored ones map to themself

This JS implementation should be a good start for a Perl port

https://github.com/rottytooth/rot8000/blob/main/rot8000.js

You can use ord and chr to convert to codepoint and back

HTH!