What you are seeing is probably cpan asking the question because the Makefile.PL has defined PREREQ_PM as in:

use ExtUtils::MakeMaker;
WriteMakefile(
    ...
    PREREQ_PM    => {
                     'DBI' => 1.00,
                     'DBD::Oracle' => 1.12,
                     ...
                    },
);
[download]

How cpan deals will pre-requisites depends on the setting of the prequisites_policy config variable. It can be set to either ask (always ask to install missing pre-requisites), follow (don't ask, always install) or ignore (don't install).