You must take care that your lookbehind, if incorporated into the same regexp, doesn't change what you match. If it's too greedy, the starting point of the former pattern might shift backwards.

So, IMO, the safest way is to use two patterns. I'm not sure if you can integrate it into one pattern, I doubt it, so that you can still simply make the whole match fail if the lookbehind fails. Two independent matches won't do that. Anyway, enough blahblah, here's my coarse idea:

my $success;
while(/PATTERN/g) {
    if(substr($_, 0, $-[0]) =~ /LOOKBEHIND\z/) {
        # got a match!
        $success = 1;
        last;
    }
}
[download]

For example:

$_ = 'bar    bar         obar     foooooobar   bar';
my($success, $start) = 0;
while(/bar/g) {
    if(substr($_, 0, $start = $-[0]) =~ /fo+\z/) {
        # got a match!
        $success = 1;
        last;
    }
}
print "$success: $start\n";
[download]

printing:

  1: 35

Using $start, the start position of the outer match, you can try again if you want, to get the captured values and @- and @+. For some odd reason, capturing @- and @+ in the loop made it loop forever. *shrug*