The POD perlretut in the section called, "Non-capturing groupings"
For sure it's covered in Learning Perl, perlre, perlretut, the Perl Cookbook, and Mastering Regular Expressions. Also by using Google with the search term, "Non-capturing regular expression grouping", the second result on the page is http://www.regular-expressions.info/refadv.html.
There's no better source on Perl's regular expressions (as well as many other flavors) than Mastering Regular Expressions, by Jeffery Friedl (O'Reilly). Perl's POD may document the very latest changes, and it is comprehensive. But spending some time with the Owls book is quite an experience.