I am using DBI with MySQL and am looking for a way to test if a query returned an empty recordset.

You might have better luck by rephrasing the question as "how many users with this name exist?"

Try something like this (untested):

  my $sth = $db1->prepare(
    "SELECT count(*) FROM users WHERE username = ?"
  );
  $sth->execute($Username_POST);
  my $rs = $sth->fetchall_arrayref();
  my $count = $rs->[0]->[0];
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