[WIP] Begin to distinguish between Perl warnings and errors (#933)

* If a Perl script compiles, there are only warnings and no errors

* Let the first Perl error or warning win.

Take the following example:

***

sub foo {
    my $thing;

***

This might have the following messages when we compile it:

Missing right curly or square bracket at warning.pl line 7, at end of
line
syntax error at warning.pl line 7, at EOF
warning.pl had compilation errors.

With the current behaviour, we just get a "syntax error" message, which
isn't all that helpful.  With this patch we get "Missing right curly or
square bracket".

* Fix variable scope and pattern matching syntax

* Use named variable to enhance clarity when matching Perl output

* Add more tests for Perl linter

* Remove unnecessary parens

* Simplify check for pattern match

ale_linters/perl/perl.vim

@@ -27,12 +27,20 @@ function! ale_linters#perl#perl#Handle(buffer, lines) abort
     let l:output = []
     let l:basename = expand('#' . a:buffer . ':t')
 
+    let l:type = 'E'
+    if a:lines[-1] =~# 'syntax OK'
+        let l:type = 'W'
+    endif
+
+    let l:seen = {}
+
     for l:match in ale#util#GetMatches(a:lines, l:pattern)
         let l:line = l:match[3]
+        let l:file = l:match[2]
         let l:text = l:match[1]
-        let l:type = 'E'
 
-        if ale#path#IsBufferPath(a:buffer, l:match[2])
+        if ale#path#IsBufferPath(a:buffer, l:file)
+        \ && !has_key(l:seen,l:line)
         \ && (
         \   l:text isnot# 'BEGIN failed--compilation aborted'
         \   || empty(l:output)
@@ -43,6 +51,8 @@ function! ale_linters#perl#perl#Handle(buffer, lines) abort
             \   'text': l:text,
             \   'type': l:type,
             \})
+
+            let l:seen[l:line] = 1
         endif
     endfor