> <@freenode_cyris21260:matrix.org> Can someone tell me what I am doing wrong here?

 * you assume that by installing the package "pxlib" using nix-env, it will somehow become available in nixpkgs, since you try to use it in the build inputs of "pxwrite" by writing "pkgs.pxlib". But that assumption is wrong. you'll have to import the package definition of "pxlib" in "pxwrite". e.g.

with import <nixpkgs> {};
let
  pxlib = import ./pxlib.nix {};
in

stdenv.mkDerivation {
pname = "pxwrite";
version = "0.0.8";
src = downloads/pxwrite_0.0.8.orig.tar.gz;
buildInputs = [ pxlib pkgs.perl ] ++ (with pkgs.perlPackages; [ libxml_perl ]);