In your two additional examples (where you put $_ into the regex's) you are defeating the purpose of the 'compiled' regex. The compile option, I thought, was specifically for cases where the regex itself does *not* change.

I thought the advantage of using the qr() construct was so that the compiler would look at the regex and do the right thing...i.e., compile the regex once if it does not change, otherwise recompile every usage when it *does* change. I'm not sure what the compiler does when you tell it to compile a regex (using the /o option specifier) that is not essentially static...in fact, I vaguely recall reading that it results in unpredictable regex compiler behavior. Is that the cause of the ...different behavior... that is reported in this thread? My recollection of the compiler behavior may just be a bad memory...since I usually work hard to ensure that the regex is really static and can safely expect to compile it only onc.

I don't consider myself to be more than a novice with regex's...I'm gaining more and more experience with them and starting to drift towards that dangerous mental construct of seeing every problem as a regex waiting to birth itself. So all of the other good advice and commentary is from much more accomplished and knowledgable monks.