I wanted to address a few of your points. I intended to do so earlier, but just didn't have the time until now.

1. You mentioned that using $_ to hold a pattern is bad style. But he's using grep. If he's passing patterns to grep what choice does he have? Remember: my @found = grep { do something with $_ } @input_list;. If he really intended grep { $something =~ m/$_/ } @array; do you know of some other way to use grep that wouldn't involve $_?

   Your sub-item (a): The default behavior of the m// operator is to match against $_, but he didn't invoke its default behavior.

   Sub item (b): Yup... but not in this case.

2. That's a better question: Why is he using an empty string as the pattern in a regexp? Additionally, why would he be interested in matching an empty pattern against an empty string literal. But I have a feeling that's not the whole story. His input list to grep probably consists of a lot of patterns, which may include an empty string. Why is he matching against the literal empty string? That's again probably just a boiled down example of the problem. I suspect that where he showed us a literal empty string, there's probably a foreach loop with an iterator on the lefthand side of the match operator, as in the following snippet:

   my @patterns = ( .................. );
   foreach my $iterator ( @array ) {
       my @found = grep { $iterator =~ m/$_/ } @patterns;
       do_something_with( @found );
   }
   [download]

   You make a good point that the behavior of a m// (empty pattern match) is impossible to intuitively predict. Though the behavior may be useful in some situations, those situations probably warrant a comment in real-world code so that it doesn't initiate a head-scratching and document-reading session when someone looks at the code six months later. Regardless of whether the feature is generally familiar to people, it seems to be accurately documented. It might be a dusty corner in the halls of pattern matching, but Perl is full of dusty corners that provide useful features when nothing else would be quite as convenient.

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