The ( and ) around the .*? are grouping parentheses... they tell Perl's regular expression engine to capture whatever it matches between the [ and ], and store it in a variable: the variable name will be a digit corresponding to the set of parentheses ($1 for the first group, $2 for the second group, etc.).

So, in this case, Perl captures what it finds between the [ and ] and stores that string in $1.

And what does it match? In this case, we've told it to match any character ("."), repeated 0 or more times ("*"), in a non-greedy manner ("?"). This last is best illustrated by example. Say your original string was

    aslkj[2099asgjskjw]asljgn[awoeiwj]
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There are two sets of [ and ] strings in there. Without the "?" in the regular expression, Perl would match from the first [ to the last ], putting the following into $1:

    2099asgjskjw]asljgn[awoeiwj
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Most likely, this isn't what you want, so we put in the non-greedy modifier to make Perl do what you want. :)

By the way, I said that "." matches any character; this isn't strictly true. It usually doesn't match carriage returns; if you want it to match carriage returns, add the "/s" modifier at the end of the regular expression.

For more information, take a look at perlre. It explains all of this and more.