Register "cpanfile" as a Perl filename (#3704)

lib/linguist/languages.yml

@@ -3217,6 +3217,8 @@ Perl:
   - ".pod"
   - ".psgi"
   - ".t"
+  filenames:
+  - cpanfile
   interpreters:
   - perl
   language_id: 282

samples/Perl/filenames/cpanfile

@@ -0,0 +1,11 @@
+requires 'perl', '5.008001';
+
+requires 'JSON';
+requires 'Path::Class', 0.26;
+requires 'WebService::Dropbox', 2.06;
+requires 'DateTime::Format::Strptime';
+requires 'Encode::Locale';
+
+if ($^O eq 'darwin') {
+	requires 'Encode::UTF8Mac';
+}