You're basically matching by luck. I believe Perl is interpreting the regex as follows:

/^([character-class].*?):(.*)/
#where character-class == a-zA-Z_)[3,
[download]

That is, the '[3,' is getting interpreted as part of the character class with its missing close bracket, not as some kind of modifier on the class.

Of course this isn't as strict a matching pattern as you were expecting!