What is it that 'knows' that the module is not as originally installed and HOW does it know?

It does not.

And it doesn't produce the error message "Spreadsheet::ReadSXC not installed at ..."

It produces the message "Can't locate Spreadsheet/ReadSXC.pm in @INC".

That's because Perl looks for a file named Spreadsheet/ReadSXC.pm relative to a directory in @INC.

Again, it doesn't check if the file has been "changed".

@INC is influenced by env var PERL5LIB.