More specifically I think that you either need to ban most uses of map and grep, or you need to make it a policy that it is the responsibility of the function writer to not pollute global variables needlessly, in particular not to pollute $_. It is the responsibility of the user of the code to test specifically for that (if you have testing suites, it should be properly tested there). But it is the job of the writer of a function to be sure that their functions are good citizens.

If you are not willing to make this be your policy then I strongly recommend that you ban most natural uses of map and grep from your code base for your sanity. Oh, and I would prefer to work elsewhere, thankyouverymuch.

my @foo = (2, 4, 8);
foreach my $num (@foo) {
    $num *= 2;
}
$, = ",";print @foo;print "\n";
[download]

Like Ovid says below, this is a 'problem' of aliasing. But it actually isn't a problem unless you assign to $_ (or $num, or whatever your iterator is called), which is typically a deliberate decision, and not usually something that happens by accident. Although I can imagine typos (the olde =/== switcheroo), or some weird precedence error doing this assignment. Also, if your &dosomething operates on its @_ directly, you'll be at risk; i.e., this does the same thing as the above:

my @foo = (2, 4, 8);
foreach my $num (@foo) {
    &double($num);
}
$, = ",";print @foo;print "\n";

sub double {
    $_[0] =  2*$_[0];  
    return $_[0];
}
[download]

sub double {
    my $num = $_[0];
    $num =  2*$num;  
    return $num;
}
[download]

-- Frag.

for (@isbns) {
  for (@$_->{Authors}) {
    # now $_ is something else :)
  }
}
[download]