Index: lib/redmine/syntax_highlighting.rb
===================================================================
--- lib/redmine/syntax_highlighting.rb	(リビジョン 18114)
+++ lib/redmine/syntax_highlighting.rb	(作業コピー)
@@ -85,7 +85,7 @@
           # See also: https://github.com/jneen/rouge/pull/1078
           text = text.gsub(/\r\n?/, "\n")
 
-          lexer =::Rouge::Lexer.guess_by_filename(filename)
+          lexer =::Rouge::Lexer.guess(:source => text, :filename => filename)
           formatter = ::Rouge::Formatters::HTML.new
           ::Rouge.highlight(text, lexer, CustomHTMLLinewise.new(formatter))
         end
Index: test/unit/lib/redmine/syntax_highlighting/rouge_test.rb
===================================================================
--- test/unit/lib/redmine/syntax_highlighting/rouge_test.rb	(nonexistent)
+++ test/unit/lib/redmine/syntax_highlighting/rouge_test.rb	(作業コピー)
@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+# Redmine - project management software
+# Copyright (C) 2006-2017  Jean-Philippe Lang
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+require File.expand_path('../../../../../test_helper', __FILE__)
+
+class Redmine::SyntaxHighlighting::RougeTest < ActiveSupport::TestCase
+  def test_highlight_by_filename_should_distinguish_perl_and_prolog
+    code_perl, code_prolog = <<'PERL', <<'PROLOG'
+#!/usr/bin/perl
+print "Hello, world!\n";
+PERL
+#!/usr/bin/swipl
+:- writeln('Hello, world!'),halt.
+PROLOG
+    filename = 'hello.pl'
+
+    assert_equal Rouge::Lexers::Perl, Rouge::Lexer.guess(:filename => filename, :source => code_perl)
+    assert_equal Rouge::Lexers::Prolog, Rouge::Lexer.guess(:filename => filename, :source => code_prolog)
+
+    # Rouge raises Rouge::Guesser::Ambiguous exception if it cannot decide
+    # between prolog and perl
+    assert_nothing_raised do
+      html = Redmine::SyntaxHighlighting::Rouge.highlight_by_filename(code_perl, filename)
+      assert_match '<span class=', html
+      html = Redmine::SyntaxHighlighting::Rouge.highlight_by_filename(code_prolog, filename)
+      assert_match '<span class=', html
+    end
+  end
+end