Add perl package

Add perl package, based on [work by
justintoo](https://github.com/LLNL/spack/pull/105).  He had too many
things pulled into that pull request, this just adds a perl package.

Support the current releases on the past three minor branches.

Run perl's tests before installing.

Install cpanm into the core (makes building on top of this perl *much*
simpler).  Controlled by a variant.

var/spack/repos/builtin/packages/perl/package.py

@@ -0,0 +1,42 @@
+#
+# Author: George Hartzell <hartzell@alerce.com>
+# Date: July 21, 2016
+# Author: Justin Too <justin@doubleotoo.com>
+# Date: September 6, 2015
+#
+from spack import *
+
+
+class Perl(Package):
+    """Perl 5 is a highly capable, feature-rich programming language with over
+       27 years of development."""
+    homepage = "http://www.perl.org"
+    url      = "http://www.cpan.org/src/5.0/perl-5.22.2.tar.gz"
+
+    version('5.24.0', 'c5bf7f3285439a2d3b6a488e14503701')
+    version('5.22.2', '5767e2a10dd62a46d7b57f74a90d952b')
+    version('5.20.3', 'd647d0ea5a7a8194c34759ab9f2610cd')
+    # 5.18.4 fails with gcc-5
+    # https://rt.perl.org/Public/Bug/Display.html?id=123784
+    # version('5.18.4' , '1f9334ff730adc05acd3dd7130d295db')
+
+    # Installing cpanm alongside the core makes it safe and simple for
+    # people/projects to install their own sets of perl modules.  Not
+    # having it in core increases the "energy of activation" for doing
+    # things cleanly.
+    variant('cpanm', default=True,
+            description='Having cpanm in core simplifies adding modules.')
+
+    def install(self, spec, prefix):
+        configure = Executable('./Configure')
+        configure("-des", "-Dprefix=" + prefix)
+        make()
+        make("test")
+        make("install")
+
+        if '+cpanm' in spec:
+            perl_exe = join_path(prefix, 'bin', 'perl')
+            perl = Executable(perl_exe)
+            cpanm_installer = join_path(self.package_dir, 'cpanm-installer.pl')
+            cpanm_package_spec = 'App::cpanminus' + '@' + '1.7042'
+            perl(cpanm_installer, cpanm_package_spec)