Something along the lines of...

my $number_of_levels = 5;
my $query = 'T1.name AS lev1';
my $join = '';
my $old_table = 'T1';
for(2..$number_of_levels) {
  $query .= ", T$_.name AS lev$_";
  $join .= " LEFT JOIN tab AS T$_ ON T$_.parentID = $old_table.ID";
  $old_table = "T$_";
}

print $finished_query = "SELECT $query FROM tag AS T1 $join WHERE T1.p
+arentID = 0";
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