Add support for perl language server (#5003)

* Fix perl test case clean-up
* Add support for perl language server

Co-authored-by: cos <cos>

test/linter/test_perl.vader

@@ -12,3 +12,5 @@ Execute(Overriding the executable and command should work):
   let b:ale_perl_perl_options = '-w'
 
   AssertLinter 'foobar', ale#Escape('foobar') . ' -w %t'
+  unlet b:ale_perl_perl_executable
+  unlet b:ale_perl_perl_options