It's also lexicalized in every loop, so writing to it doesn't affect anything else. The only time I can think of you would have to worry about this is if some function modified $_ without any loop, and I would consider that a bug (similar to any function that set a static class variable indiscriminately).

For example, the following code performs as expected, and prints "123"

    use 5.016;
    use warnings;
    my @a = qw(1 2 3);
    for (@a) {
        my @b = qw(4 5 6);
        for (@b) {
            $_ = 1;
        }
        print;
    }

This code works (prints "123") fine as well:

    use 5.016;
    use warnings;
    sub test {
        my @b = qw(4 5 6);
        for (@b) {
            $_ = 1;
        }
    }
    my @a = qw(1 2 3);
    for (@a) {
        test;
        print;
    }

The following causes a problem (prints "111"). Don't do this, it's stupid.

    use 5.016;
    use warnings;
    sub test {
        $_ = 1;
    }
    my @a = qw(1 2 3);
    for (@a) {
        test;
        print;
    }

The moral? Buggy libraries are buggy, don't use them, or submit a fix.