irssi: restore perl

Replaces perl functionality; adds fix to avoid picking up perl
destinations outside of Homebrew's prefix if a user has set PERL_MM_OPT
like CPAN suggests.

Discussion in Homebrew/homebrew#34685; resolves Homebrew/homebrew#34770; partially reverts ca09a39 from

Formula/irssi.rb

@@ -4,7 +4,7 @@ class Irssi < Formula
   homepage "http://irssi.org/"
   url "http://irssi.org/files/irssi-0.8.17.tar.bz2"
   sha1 "3bdee9a1c1f3e99673143c275d2c40275136664a"
-  revision 1
+  revision 2
 
   bottle do
     sha1 "5d9cf38c35b97dc6056adb77759ad930f912f5fb" => :yosemite
@@ -32,16 +32,22 @@ class Irssi < Formula
       --enable-ipv6
       --enable-true-color
       --with-socks
-      --with-perl=no
       --with-ncurses=#{MacOS.sdk_path}/usr
     ]
 
+    if build.with? "perl"
+      args << "--with-perl=yes"
+      args << "--with-perl-lib=#{lib}/perl5/site_perl"
+    else
+      args << "--with-perl=no"
+    end
+
+    # confuses Perl library path configuration
+    # https://github.com/Homebrew/homebrew/issues/34685
+    ENV.delete "PERL_MM_OPT"
+
     args << "--disable-ssl" if build.without? "openssl"
 
-    # It'd be nice to stick Perl support back in at some point but right now
-    # even explicitly setting a Perl libdir gets ignored by configure
-    # and it attempts to dump things in $HOME, causing permission hell. See:
-    # https://github.com/Homebrew/homebrew/issues/34685
     system "./configure", *args
     # "make" and "make install" must be done separately on some systems
     system "make"