perly.y: add $$ = 0 for midrule code blocks

perly.y: add $$ = 0 for midrule code blocks

In places where a rule contains multiple code blocks, ensure that
$$ is assigned a valid value at the end of midrule blocks, so that

    valgrind ./perl -Dpv ...

doesn't display zillions of

    Conditional jump or move depends on uninitialised value

errors, when perl tries to display the parse stack.

I've only done the various newish top-level grammar entries - these all
seemed to have the same defect, while from a quick glance elsewhere in the
file, it seemed like older rules already do this.