The easiest way to contribute is to file bugs and feature requests on JIRA.
If you would like to help code the driver, read on.
You can choose a bug on JIRA and fix it. Make a comment that you are working on it, to avoid overlap.
We would be grateful for any and all tests people would like to write.
If you think a feature is missing from the driver, check on IRC or the mailing list, then go ahead and create a JIRA case and add the feature.
To make sure your changes are approved and to speed things along:
Bonus (for C programmers, particularly):
The important files:
| perl_mongo.c # serialization/deserialization | mongo_link.c # connecting to, sending to, and receiving from the database - lib - MongoDB | Connection.pm # connection, queries, inserts... everything comes through here | Database.pm | Collection.pm | Cursor.pm | OID.pm | GridFS.pm - GridFS | File.pm - xs | Mongo.xs | Connection.xs | Cursor.xs | OID.xs