[kbenson]

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.

[btilly]

It is not lexicalized, it is localized.

And yes, I both fixed my coworker's code to not use $_, and submitted a patch to the CPAN library with the bug. But the experience taught me to be cautious about $_.

[kbenson]

Sorry, that's correct. I have lexical on the brain.

I just think saying not to use $_ in real programs is a bit extreme. Truthfully, I don't use it in for loops at all, but my code invariably uses a lot of maps and greps. To throw away the expressiveness of map and grep because there may be a problem in some third party library I use at some point is too high a price for me. Especially because by the nature of how I use map and grep I think I'm less likely to run into problem code in those cases, and if I do I think it will be fairly obvious.