The lack of any documentation for something as low-level and critical makes me not want to use this.
From a quick look at the XS source code it looks like it's Linux-only. If so, Linux::Defrag would be a better name.
All the interesting code is in the bin/defrag script: why aren't they in the module instead? It would be useful to have a module with a single defrag_file() function for instance.
The defrag_files() function to defrag all files uses a homegrown directory traversing scheme instead of File::Find.
I would expect a good defragmentation scheme would do a bit of analysis to see which files needed defragmenting (based on how fragmented files were, how often they are accessed or how critical they are, etc.). This version doesn't seem to do any of that.