$ perl -E '
    use warnings;

    for my $f ("a" .. "b") {
        my $max = $f;

        for my $i (1 .. $max) {
            say "\$f[$f] \$i[$i]";
        }
    }
'
Argument "a" isn't numeric in foreach loop entry at -e line 7.
Argument "b" isn't numeric in foreach loop entry at -e line 7.