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/strings.nix

@@ -126,6 +126,22 @@ rec {
   */
   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.