perl-mode: Display here-docs as strings instead of comments

perl-mode: Display here-docs as strings instead of comments

* lisp/progmodes/perl-mode.el
(perl-syntax-propertize-function): Handle HERE doc starter
lines ending in a comment.
(perl-heredoc): New face for HERE docs, inheriting from
font-lock-string-face.
(perl-font-lock-syntactic-face-function): Apply the new face
to HERE docs (Bug#23461).

* test/lisp/progmodes/cperl-mode-tests.el
(cperl-test--run-bug-10483): Skip for Perl mode.  The test
explicitly calls a function of CPerl mode.