$f = $pop->waitfor('//');
print "$f\n";
[download]

From the documentation, waitfor just returns success or failure in scalar context. I'd say you probably want to call waitfor in list context:

($f) = $pop->waitfor('//');
print "$f\n";
[download]

... or even:

($f, $match) = $pop->waitfor('//');
print "$f$match\n";
[download]