We have had so many hard to trace bugs because developer A changed some variable (sometimes inadvertently) that then screwed developer B...The developer who went and made the DS writable was certainly breaking the rules.

Setting the data structure writable can't be done "inadvertently".

If it happens once; you put it down to 'didn't know better', admonish the guy and move on.

If it happens twice; you put it down to 'thinks he know better', give the guy a written formal warning that any repetition will be considered deliberate sabotage, and result in both dismissal, and reporting to the authorities with a view to criminal prosecution.

And more specifically I was trying to learn some details about the Internals package and what might happen if I start messing with it.

Anything you can do--your malicious programmer can undo. Far better to be rid of the problem than chase a protracted technological arms race.

And regardless of whether the dev who changed the DS +r was willfully "breaking the rules" or merely ignorant thereof, you have a discipline problem; not a Perl problem, IMO. That's an opinion furthered by your reference to "finger pointing and hate mongering" -- as that's hardly a sign of community values that will help your organization achieve its goals.

In short, I have to go with BrowserUk's observation, "the threat of the sack...," or, in Americanized form, "Someone with sufficient authority to do so (you?) needs to tell this sucker that you won't need and won't use his/her services if s/he fails to respect the organization's rules and standards."

Life, human and corporate, is too brief to waste time on aggravations like this.

I was trying to learn some details about the Internals package

The functions are implemented in universal.c.