The flow for a dbi query that returns data is:
1. Define statement
2. Prepare statement
3. Execute statement
4. Fetch results of statement

You're missing step 3. You're code should look like this:

sub _get_billing_plan_options {
  my $self = shift;
  my @billing_plan_options;
  my $sql = $self->{'cfg'}->param("sql.get_svc_plan_options");
  $self->log('DEBUG','->_get_billing_plan_options() says our query is:
+ ' . $sql);
  my $sth = $self->{'dbh'}->prepare($sql);
  $self->log('DEBUG','Our $sth is: ',$sth);
  #you have to execute the prepared query!
  $sth->execute();
  while (my $options = $sth->fetchrow_hashref()){
    $options->{'plan_type'} =~ s/_/ /;
    push @billing_plan_options, [ 'service_plan_' . $options->{'svc_pl
+an_id'}, $options->{'plan_name'}, $options->{'plan_type'} ];
    push @billing_plan_options, [ 'service_plan_prepay_' . $options->{
+'svc_plan_id'}, $options->{'plan_name'}, $options->{'plan_type'} ];
  }
  #Don't forget to finish the statement handle
  $sth->finish;
  return \@billing_plan_options;
}
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