Zaxo suggested a particularly neat way to get all the lines containing the address you want. Which is indeed what I'd look for too. But if you really want the index instead, just change that code to

my @indexes = grep { -1 != index $contents[$_], $address } 0..$#conten
+ts;
[download]

Of course this is much less neat, but then you may adapt any of these techniques to your own needs...