There are already several modules on CPAN to do recursive data structure
merging, like Data::Merger and Hash::Merge. Data::ModeMerge differs in that it
offers merging "modes" and "options". It provides greater flexibility on what
the result of a merge between two data should/can be. This module may or may not
be what you need.

One application of this module is in handling configuration. Often there are
multiple levels of configuration, e.g. in your typical Unix command-line program
there are system-wide config file in /etc, per-user config file under ~/, and
command-line options. It's convenient programatically to load each of those in a
hash and then merge system-wide hash with the per-user hash, and then merge the
result with the command-line hash to get the a single hash as the final
configuration. Your program can from there on deal with this just one hash
instead of three.

In a typical merging process between two hashes (left-side and right-side), when
there is a conflicting key, then the right-side key will override the left-side.
This is usually the desired behaviour in our said program as the system-wide
config is there to provide defaults, and the per-user config (and the
command-line arguments) allow a user to override those defaults.