Fix #1866 - Handle empty output from Perl

ale_linters/perl/perl.vim

@@ -14,6 +14,10 @@ let s:begin_failed_skip_pattern = '\v' . join([
 \], '|')
 
 function! ale_linters#perl#perl#Handle(buffer, lines) abort
+    if empty(a:lines)
+        return []
+    endif
+
     let l:pattern = '\(.\+\) at \(.\+\) line \(\d\+\)'
     let l:output = []
     let l:basename = expand('#' . a:buffer . ':t')

test/handler/test_perl_handler.vader

@@ -7,6 +7,11 @@ After:
   call ale#test#RestoreDirectory()
   call ale#linter#Reset()
 
+Execute(The Perl linter should handle empty output):
+  call ale#test#SetFilename('bar.pl')
+
+  AssertEqual [], ale_linters#perl#perl#Handle(bufnr(''), [])
+
 Execute(The Perl linter should ignore errors from other files):
   call ale#test#SetFilename('bar.pl')