I supported Text::CSV_XS back to 5.005 until the maintenance cost to keep 5.005 alive was outweighing the cost of fixing bugs and/or adding features. (I dropped 5.005 support in June 2012)

Having 5.8.4 as a minimum is a good starting point if you require working Unicode and/or 64bit operations, pointers and ints. Unicode before 5.8.4 is buggy. Really buggy.

Even though Devel::PPPort eases porting for XS over to older versions, supporting 5.6.x really requires extra steps in the XS code for certain operations and or features, and requiring ifdef forests is no fun.

Another reason to have a minimum required version is the minimum version officially supported by the toolchain. At the moment of writing, that version is 5.8.1.

And of course, a good reason to raise the minimum is when a module you depend on, e.g. DBI, raises the minimum to a version higher than your minimum, so supporting a lower version is pointless (if you require that newer version of the module that raised the version).