Improve HERE document detection in perl-mode

Improve HERE document detection in perl-mode

* lisp/progmodes/perl-mode.el (perl-syntax-propertize-function):
Detect indented HERE documents when using a bare identifier.
(perl--syntax-exp-intro-keywords): Recognize HERE documents that come
after die, warn and eval.
(perl--syntax-exp-intro-regexp): Identify HERE documents when printing
to a filehandle with printf? and when they appear after a fat comma.

* test/lisp/progmodes/cperl-mode-resources/here-docs.pl: Add more tests.