Firstly, I don't see a config parameter called vendorarchexp. I am using the perl packaged with Debian. Maybe vendorarchexp is only set when perl has been built with vendor specifics. Here's the tail of my Config.pm:

Some time ago, most of the config stuff was moved out of Config.pm to save load time/memory. Only the most popularly inspected options are left. But you can still request $Config::Config{'vendorarchexp'} or just do perl -V:vendorarchexp.