This is probably the best way to go. Build your module with both the underlying power functionality and the higher level soft stuff and then more people can use it.
Of course, if somehow the 'simple' API functions don't justify the bulk of the interface, you might want them separated. (Rarely the case, in what I've seen. Using XML::Simple requires XML::Parser (ah, memory...I almost think you can get away with just using some other XML module, but then you still have to download something more complex anyway, kind of a moot point...)) I believe the reason XML::Simple is a separate module is to make the implementation simpler, not to save overhead.
I'm really just guessing, though.
You are what you think.