Especially Aristotle's surprising remark that no Perl construct implicitly localizes $_ at all!)

Hmm. I understand Aristotle's point, but I think his assertion is wrong.

P:\test>perl -le"$_='old value';@a=1..10; for(@a){ $_++ }; print;"
old value
[download]

Here, for is aliasing $_ to each of the values in @a, but it localises $_ first! Hence, after the loop, $_ regains it's former value.

Perl could implicitly localise $_ for the duration of a while loop in the same way giving this:

P:\test>type junk.dat|perl -e"$_='old value';{local$_; while(<>){print
+};}print;"
line 1
line 2
line 3;
old value
[download]

instead of this:

P:\test>type junk.dat|perl -e"$_='old value';while(<>){ print};print;"
line 1
line 2
line 3;
[download]

As for why it doesn't, that's a question for those with the historical perspective.