I didn't say anything that contradicts that. It's perfectly fine to use $! immediately after unlink returns an error as I showed in the very post you quoted.

In moritz's code, $! isn't being used immediately after unlink returns an error. In decreasing likeliness of occurrence,

• It's sometimes used after stat (-e) successfully returns false. (Can change $!.)
• It's sometimes used after stat (-e) returns an error. (Always sets $!.)
• It's sometimes used after unlink returns success. (!!!)