/path/to/write-nix-expression.sh Group::Module::Submodule

It will output Nix assignment formatted for placing into 
all-packages.nix amongst all the perl packages, like perlAlgorithmDiff.
Actually it will grab distribution unit Group-Module-Submodule via
search.cpan.org, and make some effort to write correct dependencies.
If among requirements there are some that have empty META.yml, or if 
any of requirements acannot be installed by just getting distribution
unit whose name can be guessed by replacing :: with -, manual editing
will be needed. 
