diff --git a/var/spack/repos/builtin/packages/perl/package.py b/var/spack/repos/builtin/packages/perl/package.py
index 51cfc8869b7..1825832644c 100644
--- a/var/spack/repos/builtin/packages/perl/package.py
+++ b/var/spack/repos/builtin/packages/perl/package.py
@@ -190,15 +190,19 @@ def setup_dependent_package(self, module, dependent_spec):
            perl('Makefile.PL','INSTALL_BASE=%s' % self.prefix)
         """
 
-        # perl extension builds can have a global perl executable function
-        module.perl = self.spec['perl'].command
-
-        # Add variables for library directory
-        module.perl_lib_dir = dependent_spec.prefix.lib.perl5
-
-        # Make the site packages directory for extensions,
-        # if it does not exist already.
+        # If system perl is used through packages.yaml
+        # there cannot be extensions.
         if dependent_spec.package.is_extension:
+
+            # perl extension builds can have a global perl
+            # executable function
+            module.perl = self.spec['perl'].command
+
+            # Add variables for library directory
+            module.perl_lib_dir = dependent_spec.prefix.lib.perl5
+
+            # Make the site packages directory for extensions,
+            # if it does not exist already.
             mkdirp(module.perl_lib_dir)
 
     @run_after('install')