Use new reserved-for-perl syntax

[khwilliamson]

One commit changes to use this less obtrusive syntax for a symbol added in 5.43.8

The other commit changes two symbols created earlier in the 5.43 series that are so short that they might collide with an XS symbol.

• This set of changes does not require a perldelta entry.

[jkeenan]

In my reading this p.r. qualifies for a 'defer-next-dev' label.

[khwilliamson]

On the contrary, it is somewhat important to get it into 5.44. It changes two names designed for internal use that are visible everywhere, and are so short that they could have some likelihood of conflicting with an existing symbol. These symbols did not exist in 5.42. No stable code can be affected by their renaming.

[tonycoz]

"handycommah" in the first commit message

[tonycoz]

I think the Perl_ is more readable than the _pl_ suffix and only one character longer.

[khwilliamson]

The problem with Perl_ as a prefix is that it is the unimportant part; existing only for reasons not related to the point of the variable. The suffix form keeps the important part first and foremost. Please consider the possibility that years of seeing the prefix form may have conditioned us to look past it without thinking. But my expectation is that someone with less experience with the way we have always done things would find the suffix form more congenial.