> Config.pm tells me that $Config{cpp} would be "the" C++ compiler to use:

Wrong, most wrong. $Config{cpp} is the C preprocessor. There is no Config.pm variable for a C++ compiler.

In most places you can just use $Config{cc}, though. The compilers are the same, really, or at least the frontend recognizes from the source code filenames (use .cpp for C++) which backend to use.