As Fletch pointed out, what you want to use is the @+ and @- variables. However, there are some gotchas I figure are worth mentioning. It's very important which variables you choose to work if you want it done properly. A correct snippet is shown below:

my @groups = map { defined $-[$_] ? substr($string, $-[$_], $+[$_] - $-[$_]) : undef } 1 .. $#+;

What's important to note is that you must use @+ to get the number of submatches, and @- to check for successfulness. (As it is now (on my activeperl 5.6.1) you can use defined $+[$_] too to check for successfulness, but the documentation says nothing about it, and only $-[$_]'s definedness is mentioned. So the safe one ought to choose to work with @- for this.)

Hope I've helped,
ihb

Update:
I just scanned through perlretut (a document I haven't read since I figured I knew regexes well enough to need a tutorial when it was written) and found that all the info you have received in this thread is concisely described in it under "Extracting matches".