my @AssignmentIDs = ();
my $GetAssignmentID_sth=$query_dbh->prepare($SQL);
$GetAssignmentID_sth->execute();
my $rows = $GetAssignmentID_sth->rows;
  if($rows > 0) {
    while (my ($AssignmentID, $UserCount) = $GetAssignmentID_sth-> fetchrow_array() )  {
      	push(@AssignmentIDs, $AssignmentID);
    }
  }
my $AssignmentIDs = join(",",@AssignmentIDs);
$GetAssignmentID_sth->finish();

##</code><code>##

if($rows > 0) {}

##</code><code>##

"my" variable $rows masks earlier declaration in same scope

##</code><code>##

undef $row;