Why should Perl special variables get a pass on that?

If you, as a "former monk" don't understand, then it both explains why you left; and why nothing I could possibly say would make the slightest bit of difference to your perception of this situation.

The key indicator here is your phrase: "Mutating global state is bad."

By which you mean: varying variables with top level scope. is verboten; (by you!)

1. There is no point in having variables; if you cannot vary them!

   Variables that do not; or are not "permitted" to, vary, are constants.

   (It is this, beyond the performance penalties, that make Readonly a nonsense.)

2. Every program has a 'top level scope'.

   Choosing to differentiate between "global variables" and "top level, lexical variables" is a sham. A pointless and puerile non-distinction.

   Further, every (useful) program ever written makes some assumptions about ... whatever. This is usually referred to as "configuration".

   So your (bland and uninteresting; unless you bother to follow up with some supporting argument) statement reduces to: is there any legitimate reason for a program to ever override its startup assumptions?

   And fine, if you can sit wherever you are, and hand-on-heart, conclude that there is never a legitimate reason for a program to override its startup assumptions; then you've won the argument.

   But, before you pat yourself on the back consider each of these:

   • Have you never written a program that needed to override the default assumption about what constitutes a newline?

     Eg. A *nix program that used sockets.

   • Have you never written a program that needed (or benefited from) overriding the default value of cwd?

   • Have you never written a program that benefited from overriding the default setting of STDIN, STDOUT or STDERR?

     You've never used the one-arg form of select; nor redirected STDERR to a log file?

   • You've never messed with %INC?

     Eg, used lib?

   • You've never tweaked %ENV?
   • How about installed your own signal handler in %SIG?
   • You've never felt the need to reset $@, $!, $^E, and $? before running code that might influence them; and which you need to make decisions upon the values they might acquire?

If you can say "No" to all of those; then you've both won the argument, and are either a liar; or not a programmer.

Being selective about what functionality you accept as legitimate, and what you personally choose to reject is one thing; attempting to impose your affectations upon the world at large is another.