The first para of this section of perlsyn is relevant here: https://perldoc.perl.org/perlsyn#Foreach-Loops, specifically:

If the variable was previously declared with my, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localization occurs only for non C-style loops.

So the variable is localised inside the loop, but your sub wrapped over the original, non-localised version which is not updated.

It's a bit like generating a new my $i inside a new sub, e.g. inner_sub the code below.

There is perhaps an issue of clarity in the docs given one cannot call local on a lexical variable but that text might pre-date the introduction of my declarations (and localization might not be referring to the local keyword).

#!perl

use strict;
use warnings;

our $i = 0;

for (1 .. 3) {
  show();
}

inner_sub();

sub inner_sub {
    my $i = 8;
    show();
};

sub show {
  print "$i\n";
}
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