IMHO an xml document is not a database. XML is used for exchanging data. You probably want to look at mySQL or PostgreSQL for database functionality.

Having a real database will allow you to categorize all the sheets of music by genre, author, etc and allow your site's users to search or filter based on the parameters you decide to support.

You may decide to use XML for encoding the actual sheet of music while storing into the database. ie. one xml document per guitar tab. I do not know if someone already has a standard DTD for your use, or you might have to roll your own.

You will need DBI and DBD to access the database, XML::Writer to write the XML document and XML::Parser to read it back in.

You could probably eliminate the xml piece altogether and just store the guitar tab in whatever format you use now.