What I’ve got so far (in my fork) is a test file that uses Text::VimColor to generate HTML and compare it against a reference HTML document to determine if the syntax file is doing its job. If a reference file can’t be found, it will create it and skip that test. Here’s what it looks like:

$ make test
prove -rv t
t/01_highlighting.t ..
ok 1 - Correct output for t_source/perl/basic.t
ok 2 # skip Created t_source/perl/advanced.t.html
ok 3 - Correct output for t_source/perl6/basic.t
1..3
ok
All tests successful.

In case of failure, it will use Test::Differences to show you what’s wrong, and write the incorrect output to disk for you to inspect:

$ vim syntax/perl6.vim # make a bad change
$ make test
prove -rv t
t/01_highlighting.t ..
ok 1 - Correct output for t_source/perl/basic.t
ok 2 - Correct output for t_source/perl/advanced.t
not ok 3 - Correct output for t_source/perl6/basic.t

#   Failed test 'Correct output for t_source/perl6/basic.t'
#   at t/01_highlighting.t line 77.
#
«output from Test::Differences showing the offending lines»
# You can inspect the incorrect output at t_source/perl6/basic.t_fail.html
1..3
# Looks like you failed 1 test of 3.
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/3 subtests

The only big downside to this is that 01_highlighting.t tests all the source files in one go. You currently can’t tell it to only test one specific file.