A better idea, don't prompt at all, no matter what :)

Simply document the appropriate enviroment variable

 $ENV{PERL_MYCPANMODULE_CONFIG}='foo.yaml';
$ENV{PERL_MYCPANMODULE_USERPASS}='user:pass';
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Or Makefile.PL switch

perl Makefile.PL --userpass=user:pass
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and then Makefile.PL stores that in MYCPANMODULE.ini next to Makefile.PL for use during  make test or some such

I believe Module::Build provides something out of the box for this way, I might be wrong