$ perl -E '
    use strict;
    use warnings;

    for $i (1,2,3) {
        say $i;
    }

    my $i;
    say $i // "undefined";
'
Global symbol "$i" requires explicit package name (did you forget to declare "my $i"?) at -e line 5.
Global symbol "$i" requires explicit package name (did you forget to declare "my $i"?) at -e line 6.
Execution of -e aborted due to compilation errors.