[btilly]

The problem is that $_ is global and always in package main.

A specific problem that fixed this in my memory was a bug in my co-worker's code which boiled down to his looping over an array with $_, then calling a function in his loop that was in a CPAN library that assigned to $_. The result was that he wiped out his whole array.

Whenever you are calling out to code that someone else wrote, you don't know if issues like that could exist, and therefore it isn't safe to use $_. Conversely if you're writing code that someone else will call, be a good citizen and don't assign to $_ either implicitly or explicitly.

[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.