initialization
execute an SQL statement
while (my $aref=$sth->fetchrow_arrayref) {
    ... write to spreadsheet ...
    prepare & execute an SQL statement
    while (my $aref=$sth->fetchrow_arrayref) {
        ... write to spreadsheet ...
        prepare & execute an SQL statement
        while (my $aref=$sth->fetchrow_arrayref) {
            ... write to spreadsheet ...
            prepare & execute an SQL statement
            while (...) {
               <<< same thing for several more levels
            }
        }
    }
}

##</code><code>##

sub add_aref_to_sheet {
    my ($aref, $worksheet0) = @_;

    my ($fname, $lname, $title, $supervisor) = @$aref;

    #write data to spreadsheet row by row 

    $worksheet0->write($row++, 2, $fname, $format_HRtop2);
    $worksheet0->write($row++, 2, $lname, $format_HRmiddle);
    $worksheet0->write($row++, 2, $title, $format_HRmiddle);
    $worksheet0->write($row++, 2, $supervisor, $format_HRbottom);
    $row++;

    return $supervisor;
}

##</code><code>##

our $lname;
our $fname;
our $title;
our $eid;
our $supervisor;
our $supervisor2;
our $supervisor3;
our $supervisor4;
our $supervisor5;
our $supervisor6;
our $supervisor7;
our $supervisor8;
		
#select fname, lname, csg from acnse where supervisor ='pierre.nanterme';
my $sth = $dbh->prepare("SELECT fname, lname, title, supervisor FROM acnse WHERE eid ='employeID';");
$sth->execute or die $sth->errstr;

my $row=23;

while ( my $aref = $sth ->fetchrow_arrayref)
   {
   $supervisor2 = add_aref_to_sheet($aref, $worksheet0);

   my $sth = $dbh->prepare("SELECT fname, lname, title, supervisor FROM acnse WHERE eid ='$supervisor2';");
   $sth->execute or die $sth->errstr;

   while ( my $aref = $sth ->fetchrow_arrayref)
      {
      $supervisor3 = add_aref_to_sheet($aref, $worksheet0);

      my $sth = $dbh->prepare("SELECT fname, lname, title, supervisor FROM acnse WHERE eid ='$supervisor3';");
      $sth->execute or die $sth->errstr;
			
      while ( my $aref = $sth ->fetchrow_arrayref)
         {
         $supervisor4 = ....
         ... continue for several more levels ...
         }
      }
   }

sub aref {
   ... same as shown earlier ...
}