If I had to do this in Perl, I would look at what the database vendors have to offer. Oracle and IBM both sell products for two-phase commit across multiple databases from different vendors. They also both implement the XA standard.

Frankly, it doesn't sound that hard to build a very basic transaction monitor that would do two-phase commit across XA databases, probably tracking state with a local transaction-capable database like SQLite. The thing is, it's probably not worth the trouble to build it. There is just so little call for this sort of thing in real-world applications. Most companies are trying to do simpler things, like basic replication. When something comes along that could conceivably be done with two-phase commit, even the DBAs typically choose to go with something less safe but much simpler. I suspect the main customers for distributed transactions are the financial industry.