$client_descr->{"Handle"} = $handle;
$client_descr->{"In"}     = \$in;
$client_descr->{"Out"}    = \$out;
$client_descr->{"Err"}    = \$err;

# later ...
my ($handle, $in, $out) = ($client_descr->{"Handle"}, $client_descr->{"In"}, $client_descr->{"Out"});
$in .= "new data\n";
pump $handle until $$out =~ /data arrived:/; # note the dereferencing - $$out
print $$out;