You can split split, but it may not be the best solution:

use strict;
use warnings;

while (<DATA>) {
    my $ipaddr = (split /\s+/, (split /#/, $_)[0])[1];
    print "ipaddr = $ipaddr\n";
}

__DATA__
client 127.0.0.1#47560: query: host.example.com IN AAAA +
[download]

prints:

ipaddr = 127.0.0.1
[download]