Yes, it's possible for you to upload a new version of Math::Brent to CPAN. It will be marked as an unauthorized version though, and will not be indexed. This means that although it will become part of CPAN, and carried by all CPAN mirrors, the cpan command-line client will not find it. (It can still be installed via the command-line client; you just need to provide the full path to the file, a la install JOEBLOGGS/Math-Brent-1.234.tar.gz.)

However, if you're making large scale changes to it, such that your version and the original have incompatible APIs, then it serves end users better for your module to have a different name.