So I would say that any code that *sets* $_ is, by definition, wrong.

Unnecessary, perhaps, but not wrong. Setting $_ can be useful:

1. As a topicalizer in a single-item for loop, to make a series of operations on a common value read more cleanly.

   Instead of this:

   $h->{foo}{bar} =~ s/[._]/ /g;
   Some::Custom::Function($h->{foo}{bar});
   $h->{foo}{bar} =~ s/^\s+//;
   $h->{foo}{bar} =~ s/\s+$//;
   $h->{foo}{bar} =~ s/`/'/g;
   [download]

   You can write this:

   for ($h->{foo}{bar}) {
       s/[._]/ /g;
       Some::Custom::Function($_);
       s/^\s+//;
       s/\s+$//;
       s/`/'/g;
   };
   [download]

2. And the occasional weird circumstance, like conveniently modifying the values of a hash:

   map { $_ = foo( $bar, $_ ) } values %somehash;
   [download]

On the other hand, where real for loops are concerned, I find it reduces confusion to always provide your own lexical topic (e.g.  for my $key (@keys) { ... } ). A bit backwards from the way the language is designed, I suppose.