This module has now been deprecated: support.braintreepayments.com/custome... Unfortunately, since the release contains no real documentation, this very important change in status has basically been left undocumented.
If you dig deeply enough, you'll find that it's documented _not_ in the README, but in README.md
Perl has a standard interface for payment gateways: Business::OnlinePayment. This module (presumably, there is no documentation) implements its own one-off interface instead. It is the equivalent of a database access module that isn't a DBD::
This module is a Perl interface for the Braintree Payments service.
As another reviewer noted, there's no documentation in the package itself. It is, however, reasonably well documented on the vendors website (the module is useless without their website anyway): www.braintreepayments.com/docs/perl -- at least a pointer to that would be nice to have inline.
The API is nicely "Perl-ified" while still following their general API conventions (so the "base documentation" and examples in other languages are useful, too).