Fix for RT #118169 .

https://rt.perl.org/rt3/Public/Bug/Display.html?id=118169

subroutine arguments are no longer shown in perl debugger ( x @_ in perl
-d) .

Regression from perl-5.16.x due to the perl -d refactoring. Fixed with a
test.
A more robust test for the output of x.

For the last commit.
Fix the mutability of @_ in perl -d.

With a test.

See Father C.'s comment on RT #118169.