diff --git a/runtime/lua/vim/filetype/detect.lua b/runtime/lua/vim/filetype/detect.lua
index 95e55ebb21..1fe7db0e1f 100644
--- a/runtime/lua/vim/filetype/detect.lua
+++ b/runtime/lua/vim/filetype/detect.lua
@@ -1116,6 +1116,8 @@ function M.perl(path, bufnr)
   end
 end
 
+local prolog_patterns = { '^%s*:%-', '^%s*%%+%s', '^%s*%%+$', '^%s*/%*', '%.%s*$' }
+
 --- @type vim.filetype.mapfn
 function M.pl(_, bufnr)
   if vim.g.filetype_pl then
@@ -1124,11 +1126,7 @@ function M.pl(_, bufnr)
   -- Recognize Prolog by specific text in the first non-empty line;
   -- require a blank after the '%' because Perl uses "%list" and "%translate"
   local line = nextnonblank(bufnr, 1)
-  if
-    line and line:find(':%-')
-    or matchregex(line, [[\c\<prolog\>]])
-    or findany(line, { '^%s*%%+%s', '^%s*%%+$', '^%s*/%*' })
-  then
+  if line and matchregex(line, [[\c\<prolog\>]]) or findany(line, prolog_patterns) then
     return 'prolog'
   else
     return 'perl'
@@ -1232,11 +1230,7 @@ function M.proto(_, bufnr)
   -- Recognize Prolog by specific text in the first non-empty line;
   -- require a blank after the '%' because Perl uses "%list" and "%translate"
   local line = nextnonblank(bufnr, 1)
-  if
-    line and line:find(':%-')
-    or matchregex(line, [[\c\<prolog\>]])
-    or findany(line, { '^%s*%%+%s', '^%s*%%+$', '^%s*/%*' })
-  then
+  if line and matchregex(line, [[\c\<prolog\>]]) or findany(line, prolog_patterns) then
     return 'prolog'
   end
 end
diff --git a/test/old/testdir/test_filetype.vim b/test/old/testdir/test_filetype.vim
index aa522c3098..ba7836cd4f 100644
--- a/test/old/testdir/test_filetype.vim
+++ b/test/old/testdir/test_filetype.vim
@@ -2600,6 +2600,44 @@ func Test_pro_file()
   call assert_equal('prolog', &filetype)
   bwipe!
 
+  " IDL
+  call writefile(['x = findgen(100)/10'], 'Xfile.pro', 'D')
+  split Xfile.pro
+  call assert_equal('idlang', &filetype)
+
+  filetype off
+endfunc
+
+
+func Test_pl_file()
+  filetype on
+
+  "Prolog
+  call writefile([':-module(test/1,'], 'Xfile.pl', 'D')
+  split Xfile.pl
+  call assert_equal('prolog', &filetype)
+  bwipe!
+
+  call writefile(['% comment'], 'Xfile.pl', 'D')
+  split Xfile.pl
+  call assert_equal('prolog', &filetype)
+  bwipe!
+
+  call writefile(['/* multiline comment'], 'Xfile.pl', 'D')
+  split Xfile.pl
+  call assert_equal('prolog', &filetype)
+  bwipe!
+
+  call writefile(['rule(test, 1.7).'], 'Xfile.pl', 'D')
+  split Xfile.pl
+  call assert_equal('prolog', &filetype)
+  bwipe!
+
+  " Perl
+  call writefile(['%data = (1, 2, 3);'], 'Xfile.pl', 'D')
+  split Xfile.pl
+  call assert_equal('perl', &filetype)
+
   filetype off
 endfunc