$query = "select * from finalLevel3 where id=\"$ID\";";
$sth = $dbh -> prepare($query);

# I would suggest this:
my $query = $dbh->prepare("SELECT * FROM finalLevel3 WHERE id='$ID'");

##</code><code>##

 foreach $item (@{$clusters}) {
 #.....
 }

##</code><code>##

 
foreach my $cluster (@$clusters) {                                 
#.....
}

##</code><code>##

 
my @list = @{listRefReturningFunction()};
#also consider:
my $r = {"a" => [1,2,3,4],
         "b" => [5,6,7,8],
        };
print @{$r->{"a"}}; #prints 1234
                    #here print @$r->{"a"} won't work
                    #because this is a subscripted expression
# now with list...
my @x =(1,2,3,4);
my $xref=\@x;
print @$xref;       #prints 1234 also
                    #this works because there is no subscript