Agreed, but he could copy $_ and define fall-backs for edge cases like "". (I'm not sure if he should)

grep { my $pat = ( $_ eq ="" ? EMPTY_PAT : $_ ); 
       $string =~ m/$pat/g
     } @patterns;
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If this gets more complicated he should consider using a function instead of an anonymous block.

2. > I suspect that where he showed us a literal empty string, ...

I suppose he is checking multiple log-files simultaneously for the same matching patterns. By grepping patterns, he is calculating the intersection of all patterns which apply to all files.

> those situations probably warrant a comment in real-world code

That's why I would prefer a special var $PATTERN for this behavior.

UPDATE: what bothers me is not this special behavior of a literally empty match m// but that of "If the PATTERN evaluates to the empty string". IMHO that $pat="";m/$pat/ behavior is difficult to justify.