There's a DB.pm that's part of the debugger. There are also DB.pm files for Foomatic (for its driver database) and for Devel::Cover (for access to a database of code coverage info). I don't see anything strictly database-related by that name. Anything here using DB.pm uses it for some project's own uses for a database.

I do have a db.pm and a Db.pm in various projects stored on my system. I wrote both, with the latter being a newer version of the former. These are related to the specific database layer I use for certain projects, but you wouldn't have that.

I also have a Db.pm on my hard drive that's part of the storage layer for the Dada mailing list software. It has no _login method or sub, though.

I can't think of a globally applicable way to automatically find the origin of a file, but the http://search.cpan.org site has for each module distribution a link to a file named MANIFEST which most any module distribution should have. DBD::mysql doesn't come with a db.pm file per its manifest. Perhaps you could search all files named MANIFEST in your CPAN build directory to see if they mention a db.pm file.

Is there any copyright info or POD in the file in question? Perhaps you could track something down that way. I certainly can't tie the file in question to a specific distribution from the information in your node.