Still, I'm still curious if there is some way I can force Perl into a pre-Unicode state.

There's no way to disable support for wide strings. The closest is use bytes;, but it would have made your error harder to find.

There's no way to prevent chr(0x4096) from working short of overriding chr.

$ perl -E'
    use subs qw( chr );
    sub chr(_) {
       my $ch = CORE::chr($_[0]);
       $ch lt "\xFF"
           or die;
       return $ch;
    }
    say chr for 65, 4096;
'
A
Died at -e line 5.
[download]

My guess is that in the past, attempting to give chr() an argument greater than 255 would have resulted in an fatal error (or at least warning) that would have tipped me off to the problem.

It did result in a fatal error, and despite being further down the line, the error message was spot on.