For special output units file and target (see Output Units), the function reference is:
($target, $file) special_unit_target_file_name ($converter, \%output_unit, $default_target, $file_name) ¶$converter is a converter object. \%output_unit is the special output unit hash. $default_target is the target that has been already set, and $file_name is the file name that has been already set. The function should return the $target and $file.
To determine the variety of the special output unit processed, the
output unit special_unit_variety hash key can be used.
See Table 16.1.