I just wonder since, after all, it does what I mean without warnings and my

Because without strict and warnings, the variable is a package variable, not a lexical variable. The strict version of your last example would be:

use strict;
use warnings;

use vars qw($j); # or "our $j;"

$j = $_ for (1 ..3);
($j) ? print $j : warn "no warnings - missing";
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