I'm not sure I can explain why this is, but the "?" at the end of the regex is unnecessary -- in fact, it is redundant, because it's doing the same thing as the "(?:...)" operator. Get rid of the final "?" and I believe it will work. (It seemed to make a difference on a small test string that I tried.)

update: Here's a simple demonstration -- compare the outputs of these two commands:

perl -e '$_ = "one two three four"; print $1,$2,$/ if (/(one) .+?(?: t
+hree (four))/)'

perl -e '$_ = "one two three four"; print $1,$2,$/ if (/(one) .+?(?: t
+hree (four))?/)'
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For me, the first one prints "onefour", and the second just prints "one". (Anyway, I think I prefer Roy Johnson's approach.)