However, do you think this is the way it should work? After all you don't expect your variables to suddenly change as you go through a function. I know $_ is a global, but I imagine a lot of people write code like the stuff I was maintaining where there is a call to a sub in the middle of using $_. And then maybe that calls a sub and that calls another one, etc. Then someone comes along one day and puts while(<INPUT>) in one of those subs, all careful with use strict and warnings, but suddenly some bit of code miles away just breaks!

Anyway, I know I'm getting a bit hot under the collar over this, perhaps because I spent about an hour not knowing why my change had broken the whole program. But shouldn't there be some kind of health warning at least over the use of while(<>) since it must be one of the first idioms beginners learn.

I know I'm going to write

local $_;
while(<>){

}
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update No I'm not I'm going to follow jeffa's example! Doh! I might go through and put local in the old "code"/plate of spaghetti that I'm supposed to be maintaining.

-- iakobski

Yes, I think it should work that way. Then you can write code like:

    sub next_foo {
        while (<>) {
            return $. if /^FOO:/
        }
    }
 
    while (next_foo) {
        do_something # with $_
    }
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Some people won't like this style. That's fine, you don't have to. Other will like it, and Perl let's you.

-- Abigail

... or better yet, just don't use $_ in such situations. You hit the nail on the head when you mentioned someone coming along and breaking it.

my $line;
while ($line = <>) {
    # stuff
}
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i gonna wash that ambiguity outta my hair . . .

Jeff

R-R-R--R-R-R--R-R-R--R-R-R--R-R-R--
L-L--L-L--L-L--L-L--L-L--L-L--L-L--

Good explanation, Abigail. I'm just curious, is there a good reason why while (<>) doesn't implicitely localize $_?

-- Hofmator

One possible reason for why while(<>) does not implicitly localize $_ as part of its magic is that sometimes you want to access the last value of $_ outside the loop. For example:

while (<>) {
  chomp;
  last if /\S/;
}
print "You said: $_\n";
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If $_ were localized, you would have to copy the value to another variable:

my $in;
while (local $_ = <>) {
  chomp;
  $in = $_, last if /\S/;
}
print "You said: $in\n";
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Although it may just be as Abigail said, that foreach always assigns to a variable as part of the loop, whereas while can have anything in the conditional and (<>) is just a special case.