I've now posted to perl5-porters for advice. I found a smaller demonstration case that does not need CPAN.pm to illustrate the border case. Please read the posting with the title "Composing @INC from PERL5LIB". Sorry, I have no link yet but I suppose the article will appear on google groups in less than an hour.
For now the workaround for you would be to create the installation directory before starting the CPAN shell. Then perl will compute @INC as desired.