Found in /System/Library/Perl/pods/perlfaq6.pod

What's wrong with using grep or map in a void context?

Both grep and map build a return list, regardless of their context. This means you're making Perl go to the trouble of building up a return list that you then just ignore. That's no way to treat a programming language, you insensitive scoundrel!

That's in 5.6.0, but apparently it's changed in 5.8.0, go figure.