Attributes may start with underscore, and must be separated.
Thanks to Mattias Engdegård for pointing out a regex mistake.
* lisp/progmodes/cperl-mode.el (defconst): Fix bad grouping and
allow attributes to start with an underscore in
cperl--single-attribute-rx. Adjust cperl--attribute-list-rx
accordingly.
(cperl-find-sub-attrs): Allow attributes to start with an underscore.
* test/lisp/progmodes/cperl-mode-tests.el
(cperl-test-attribute-list-rx): Add new test cases for valid and
invalid attribute lists.
