You could be optimistic and assume that all dependencies will be in the Makefile.PL files (or equivalent). Of course the reality is that they should be in there anyway, and when they are not, the CPAN-Tester should fail. So while it might be optimistic, it is more how it "should be" anyway.
I would rather use existing modules such as Module::ScanDeps
to scan dependencies than to trust that a Makefile.PL or Build.PL or META.yml is accurate.
Errors from missing pre-requisites are not often posted to CPAN Testers, unless the tester notices that it's something not specified in the pre-reqs.
After all, this is for a tool to look for points of failure that the author may not be aware of.
