sub definitions within control structures don't really work how you want them to work. Your my_print function only gets defined once; not each time around the loop. If you want to redefine the sub each time around the loop, use an anonymous sub (a.k.a. closure; coderef) - which can be given a name by assigning it to a glob.

This works:

use strict;
use warnings;
no warnings 'redefine';

# stub to declare sub name, allowing it to be used
# as a bareword later on
sub my_print ();

foreach my $i (0, 1) {
    *my_print = sub () {
        print "[$i]\n";
    };
    my_print;
}
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