Add basic mode checks for perl

lib/languages/perl5.txt

@@ -0,0 +1,9 @@
+package
+use strict
+use warnings
+use v5
+use 5
+use feature
+use Exporter
+our @EXPORT
+#

test/languages/perl5_test.rb

@@ -0,0 +1,30 @@
+require "test_helper"
+module SnippetExtractor
+  module Languages
+    class PerlTest < Minitest::Test
+      def test_all
+        code = <<~CODE
+          package HelloWorld;
+          use v5.36;
+          use 5.036;
+          use strict;
+          use warnings;
+          use feature qw<say>;
+          use Exporter qw<import>;
+          our @EXPORT_OK = qw<hello>;
+
+          # test comment
+          sub hello {
+              return 'Hello, World!';
+          }
+        CODE
+        expected = <<~CODE
+          sub hello {
+              return 'Hello, World!';
+          }
+        CODE
+        assert_equal expected, ExtractSnippet.(code, 'perl5')
+      end
+    end
+  end
+end