* lisp/progmodes/perl-mode.el: Add support for indented here docs

* lisp/progmodes/perl-mode.el: Add support for indented here docs

* lisp/progmodes/perl-mode.el (perl-syntax-propertize-function):
Recognize the new <<~ syntax for indented here docs.
(perl-syntax-propertize-special-constructs): Adjust search of the
end of here docs accordingly.

* test/manual/indent/perl.perl: Add test for indented here docs.