To get absolute best performance is no easy task, requiring home-grown routines that combine generic traversal of a data structure with code generation.

If you can cope with the performance loss of using modules, then XML::Simple looks reasonable enough, although you still can't escape the formal requirement to translate your internal data structure into XML tag structure. It might be worth expressing your data structure in something like BNF as if it were a language and then writing a specific traverser that can context-sensitively call the code generation routines either in a module or in your own generation routines.