Flake for including ipcalc Perl script, its license file (as you should be doing) and the man page for it

flake.nix

{
  inputs.nixpkgs.url = "github:nixos/nixpkgs?ref=nixpkgs-unstable";

  outputs = inputs@{ flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } {
      systems = inputs.nixpkgs.lib.systems.flakeExposed;

      perSystem = { pkgs, ... }: {
        packages.default = pkgs.runCommand "ipcalc" {
          src = pkgs.fetchFromGitHub {
            owner = "kjokjo";
            repo = "ipcalc";
            rev = "0.51";
            sha256 = "sha256-260GJ0UwEdXFaINa0o8Od/nhyReNyXL8GrfKncx63jI=";
          };
        } ''
          mkdir -p $out/bin $out/share/man $out/share/doc/ipcalc

          # Documentation
          cp $src/ipcalc.1 $out/share/man/ipcalc.1

          # This seems like the correct place:
          # https://discourse.nixos.org/t/collision-between-2-license-txt-from-2-different-packages/22071/5
          cp $src/license $out/share/doc/ipcalc/LICENSE.txt

          # Hotpatch for Perl shebang
          cp $src/ipcalc ./ipcalc
          sed -i 's|#!/usr/bin/perl|#!${pkgs.perl}/bin/perl|' ./ipcalc

          # Store ipcalc executable in output
          cp ./ipcalc $out/bin/ipcalc
          chmod +x $out/bin/ipcalc
        '';
      };
    };
}

To build this, put it in a folder somewhere (or preferably a GitHub repo) and run nix build . against it. In result, there'll be two directories:

• bin (which has ipcalc in it)
• share (which has the man pages and license files)

To run it, just do result/bin/ipcalc.