For relational database access, there can be only one: DBI with the DBD module for your database. For handling the XML, depending on what you want to do with it, try XML::Simple for simple processing of well-formed documents, and XML::Parser for a full blown XML parser.
For searching the database, you can have a look at DBIx::FullTextSearch to index your XML.
These are just a few suggestions, other monks will undoubtedly have others for you to look at.