as requested: - moved function into strings.nix - renamed function from makePerl5Lib - removed duplicates entries in the resulting value - rewrote the function from scratch after learning a few things (much cleaner now)

lib/default.nix

···
     inherit (strings) concatStrings concatMapStrings concatImapStrings
       intersperse concatStringsSep concatMapStringsSep
       concatImapStringsSep makeSearchPath makeSearchPathOutput
-      makeLibraryPath makeBinPath makePerlPath optionalString
+      makeLibraryPath makeBinPath makePerlPath makeFullPerlPath optionalString
       hasPrefix hasSuffix stringToCharacters stringAsChars escape
       escapeShellArg escapeShellArgs replaceChars lowerChars
       upperChars toLower toUpper addContextFrom splitString

lib/strings.nix

···
   */
   makePerlPath = makeSearchPathOutput "lib" "lib/perl5/site_perl";
 
+  /* Construct a perl search path recursively including all dependencies (such as $PERL5LIB)
+
+     Example:
+       pkgs = import <nixpkgs> { }
+       makeFullPerlPath [ pkgs.perlPackages.CGI ]
+       => "/nix/store/fddivfrdc1xql02h9q500fpnqy12c74n-perl-CGI-4.38/lib/perl5/site_perl:/nix/store/8hsvdalmsxqkjg0c5ifigpf31vc4vsy2-perl-HTML-Parser-3.72/lib/perl5/site_perl:/nix/store/zhc7wh0xl8hz3y3f71nhlw1559iyvzld-perl-HTML-Tagset-3.20/lib/perl5/site_perl"
+  */
+  makeFullPerlPath = deps:
+    let
+      recurse = dep:
+        [ dep ] ++ dep.propagatedBuildInputs ++ (map (arg: recurse arg) dep.propagatedBuildInputs)
+      ;
+    in
+      makePerlPath (lib.unique (lib.flatten (map (arg: recurse arg) deps)))
+  ;
+
   /* Depending on the boolean `cond', return either the given string
      or the empty string. Useful to concatenate against a bigger string.
 

pkgs/top-level/perl-packages.nix

···
       checkPhase = "./Build test";
     });
 
-  # Helper function to make a PERL5LIB environment variable for a list of perlPackages and all associated runtime dependencies
-  makePerl5Lib = deps:
-    with stdenv;
-    lib.concatStringsSep ":" [
-      (lib.makePerlPath deps)
-      (lib.concatStringsSep ":" (map (dep: dep + "/lib/perl5/site_perl") (builtins.filter (dep: dep != null) (lib.flatten (map (dep: lib.getOutput "propagatedBuildInputs" dep) deps)))))
-    ]
-  ;
 
   ack = buildPerlPackage rec {
     name = "ack-2.24";