$found = 0;
foreach $i (@indata) {
  chomp($i);
  ($username,$password) = split(/\|/,$i);
  if ($in{'oldname'} eq $username) {
    $found = 1;
    <i>do your other checks for data</i>
    .
    .
    .
  }
}
if (!$found) { print "You are not registered!"; }