$wait = 2;  # wait up to this many seconds for data
# $wait = 5;
$start = time();

# zero our vars and then enter the do until loop
$timeout = $wait + time();
$data = '';
$count = 0;
do { 
    $count++;
    $data = read_data();
    # sleep 1 unless $data;
} until $data or time() > $timeout;

if ($data) {
    print "Tried $count times\nGot data '$data'\n";
} else {
    print "Tried to read_data() but failed after $wait seconds and $count tries!\n";
}

# a short sub that only returns data after 3 seconds from
# program start time
sub read_data {
  return (time() > ( $start + 3) ) ? "You have waited long enough!" : '';
}