Fix test for Perl heuristic

Improve heuristic rule for Perl6
Separate heuristic rules for .pl (with Prolog) and .pm (without Prolog)

lib/linguist/heuristics.rb

@@ -93,15 +93,23 @@ module Linguist
     end
 
     disambiguate ".pl" do |data|
-      if data.include?("use v6")
+      if /^(use v6|(my )?class|module)/.match(data)
         Language["Perl6"]
-      elsif data.match(/use strict|use\s+v?5\./)
+      elsif /use strict|use\s+v?5\./.match(data)
         Language["Perl"]
       elsif /^[^#]+:-/.match(data)
         Language["Prolog"]
       end
     end
 
+    disambiguate ".pm" do |data|
+      if /^(use v6|(my )?class|module)/.match(data)
+        Language["Perl6"]
+      elsif /use strict|use\s+v?5\./.match(data)
+        Language["Perl"]
+      end
+    end
+
     disambiguate ".ecl" do |data|
       if /^[^#]+:-/.match(data)
         Language["Prolog"]

test/test_heuristics.rb

@@ -49,15 +49,23 @@ class TestHeuristcs < Minitest::Test
     assert_equal Language["Objective-C"], match
   end
 
-  # Candidate languages = ["Perl", "Prolog"]
+  # Candidate languages = ["Perl", "Perl6", "Prolog"]
   def test_pl_prolog_perl_by_heuristics
     assert_heuristics({
       "Prolog" => all_fixtures("Prolog", "*.pl"),
-      "Perl" => all_fixtures("Perl", "*.pl"),
+      "Perl" => ["Perl/oo1.pl", "Perl/oo2.pl", "Perl/oo3.pl", "Perl/fib.pl", "Perl/use5.pl"],
       "Perl6" => all_fixtures("Perl6", "*.pl")
     })
   end
 
+  # Candidate languages = ["Perl", "Perl6"]
+  def test_pm_perl_by_heuristics
+    assert_heuristics({
+      "Perl" => all_fixtures("Perl", "*.pm"),
+      "Perl6" => all_fixtures("Perl6", "*.pm")
+    })
+  end
+
   # Candidate languages = ["ECL", "Prolog"]
   def test_ecl_prolog_by_heuristics
     assert_heuristics({