The right solution has been offered. . . but I'm a little puzzled:

#THIS LINE BELOW!
  if ( $contents =~ /^line2-(\w*)/mo ) {
    print $1; # has to be changed as well
  }
}
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Why do you use \w*? First, if all you have is 2-11, 2-12, etc., then a \d is by far better. Second, by having the * you allow this line to be matched: line2-
I assume you do not wish that to happen, so add \d+ (unless you really need the \w).

Also, as to Hofmator's optimization, I disagree. The /o will have NO effect, since the variable is not in the regex, but is bound to it. If it were $contents =~ /$line/ then it would be an optimization, but now it has no effect, and only muddies up the waters, so to say. Also, I would speculate that the /m is useless in this situation. The format is:
line1: /stuff here/
line2-2: /more stuff/
.
.
.

Therefore, I assume by what he gave us, that the information appears only at the beginning of the lines, and would not make sense to spread multiple lines; therefore taking away the /m would be better(not to mention /m doesn't optimize it, but slightly detracts from it).

UPDATE: By the way, to optimize it further, you should add the caret to the regex; /^line1/ if I am correct in that line1 is at the beginning of the line.