cperl-mode.el: Support Perl 5.38 syntax for subroutine signatures

cperl-mode.el: Support Perl 5.38 syntax for subroutine signatures

* lisp/progmodes/cperl-mode.el (defconst): New rx sequence
describing a signature with initialization.
(cperl-init-faces): integrate the new rx sequence into the
font-lock-defaults init routine (Bug#64190) (Bug#64364).

* test/lisp/progmodes/cperl-mode-resources/cperl-bug-64364.pl: Add
test data for a signature with initialization (tests indentation).

* test/lisp/progmodes/cperl-mode-resources/proto-and-attrs.pl: Add
test data for a signature with initialization (tests fontification).