If it's the same problem as the other thread, proper use of my will fix it. Don't you use use strict; use warnings;?

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

Another possibility is that there was an error performing the execute. Since you don't check for errors, did you specify RaiseError => 1?

Two other comments:

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

  can be written more simply as

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

• This one is a serious problem. Just adding quotes around the contents of $ID doesn't properly convert it from arbitrary text into an SQL string literal. You need one of the following, preferably the second:

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

  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;
  
      ...
  }
  [download]

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