My question originates from a large software project that has a complex configuration structure.

Something similar to:

# cfg1
$config =
  { "common" =>
      { debug => 1,
        logfile => "/var/log/project.log",
        path => "/opt/projects/bin",
      },
    "project_1" =>
      { name => "Project 1",
        tasks =>
          [ { name => "Prepare",
              program => "p1prep",
              arguments => [ "--init", '--default' ],
              logfile => "/var/log/p1prep.log",
            },
            { name => "Work",
              program => "p1",
              arguments => [ "--collect" ],
              logfile => "/var/log/p1.log",
              env =>
                { DISPLAY => undef,
                },
            },
            { name => "Wrapup",
              program => "signal",
              arguments => [ "p1", "finished" ],
            },
          ]
      },
    "project_2" =>
      { "and" => [ "so", "on", "..." ],
      },
  };
[download]

A particular customer could supply her own config, e.g.

# cfg2
$config =
  { "common" =>
      { debug => 0,
        logfile => "/var/log/project.log",
        path => "/opt/projects/bin",
      },
    "project_1" =>
      { name => "Project 1",
        tasks =>
          [ { name => "Prepare",
              program => "p1prep",
              arguments => [ "--init", '--default' ],
              logfile => "/var/log/p1prep.log",
            },
            { name => "Work",
              program => "p1",
              arguments => [ "--collect", "--brief" ],
              logfile => "/var/log/p1.log",
              env =>
                { DISPLAY => undef,
                },
            },
            { name => "Wrapup",
              program => "signal",
              arguments => [ "p1", "finished" ],
            },
          ]
      },
    "project_2" =>
      { "and" => [ "so", "on", "..." ],
      },
  };
[download]

But instead of supplying a full config, it suffices to supply a smaller set of modifications:

# cfgaug
$config =
  { "common" => { debug => 0 },
    "project_1" =>
      { tasks =>
          [ { name => "Prepare" },
            { name => "Work",
              arguments => [ "--collect", "--brief" ],
            },
            { name => "Wrapup" },
          ]
      },
  };
[download]

Applying #cfgaug to #cfg1 will result in #cfg2.

Since several of these augmentations are possible, the need arises to produce a single 'delta' that augments the base configuration #cfg1 to the actual state.