dws's solution works great if you only need to check if a user exists or not (although I'd use fetch() and $rs->[0] instead of fetchall_arrayref() and $rs->[0]->[0]). If you want to check if the user exists or not, and do something with the record if he does, try the following:

$rs1 = $db1->prepare("SELECT ...");
$rs1->execute(...);
unless ($row = $rs1->fetch())) {
  die("User does not exist\n");
}
... check supplied password against the one in $row ...
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