|Think about Loose Coupling|
I agree with your statements about the importance of regular expressions for a Perlist, but I have also learned a metric crapton about regular expressions in general (and Perl regexen in particular) without ever having read Mastering Regular Expressions.
Would I learn more if I read that book? Oh, certainly -- and I may well read it some day. Is it one of the core three books of a central trilogy of books that anyone getting into Perl should probably read first, specifically for purposes of learning Perl? I don't really think so. I'm of the opinion that learning enough about regexen to get by, and spending some time learning some other concepts that are also important, is more important for such a student of Perl. A deeper understanding of regexen than can be had by getting familiar with Perl in general and learning to navigate perldoc is a matter of specialization rather than of general Perl competence, I think.
I suppose your mileage may vary -- but that's kinda my point.