my $ID = ...;
    my $query = ...;
    my $sth = $dbh -> prepare($query);
    $sth -> execute;
    my @row = $sth -> fetchrow_array;

##</code><code>##

my $i = 0;
for my $item (@{$clusters}) {
   ...
   $i++
}

##</code><code>##

for my $i (0..$#$clusters) {
   ...
}

##</code><code>##

for my $i (0..$#$clusters) {
    my $ID = $units[$i][0];
    my $query = "select * from finalLevel3 where id=" . $dbh->quote($ID);
    my $sth = $dbh -> prepare($query);
    $sth -> execute;
    my @row = $sth -> fetchrow_array;

    ...
}

##</code><code>##

my $query = "select * from finalLevel3 where id=?";
my $sth = $dbh -> prepare($query);
for my $i (0..$#$clusters) {
    $sth -> execute( $units[$i][0] )
    my @row = $sth -> fetchrow_array;

    ...
}