Fix highlighting Perl in Mason files

I removed some syn statements from perl.vim that are old and unused (the
"perl_no_scope_in_variables" feature is actually implemented elsewhere in
perl.vim) and cause some things to be erroneously highlighted as function
identifiers when perl.vim is included (via "syn include") in another syntax
file (such as mason.vim).

I also added the official mason.vim to the repository, to facilitate testing.