Escape sequences \L, \U fall when we use them together

When we use escape sequences \L and \U in series we get the syntax error message:
```
perl -E 'say "\L\UXsdf"'
syntax error at -e line 1, near "\L\U"
Execution of -e aborted due to compilation errors.
```

I think it isn't a syntax error and the code above should work.

If the code shouldn't work nevertheless then the error message should be fixed.

(v5.28.1)
