There are 2 things you should be doing to make the code easier to understand and maintain. #1 is the logical use of blank lines to separate different concepts and proceses.

Illogical spacing:

  open('conf', "$PATHconf") || die "can't open $PATHconf: $!\n";
  my @conf = <conf>;
  close('conf');
  my $i=0;
  foreach(@conf){
  my $line = $conf[$i];
  
    my $t= "".$i;
    my $dbhA = DBI->connect("DBI:mysql:$VARDB_database:$VARDB_server:$
+VARDB_port", "$VARDB_user", "$VARDB_pass")
    or die "Couldn't connect to database: " . DBI->errstr;

    my $rowCount = 0;
    
    my $filename = "Output012023.xlsx";
    my $workbook = Excel::Writer::XLSX->new( $filename );
    open(FH, "<", "Debt.csv" ) or die;
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Logical spacing:

  open('conf', "$PATHconf") || die "can't open $PATHconf: $!\n";
  my @conf = <conf>;
  close('conf');

  my $i=0;

  foreach(@conf){

    my $line = $conf[$i];
    my $t    = "".$i;

    my $dbhA = DBI->connect("DBI:mysql:$VARDB_database:$VARDB_server:$
+VARDB_port", "$VARDB_user", "$VARDB_pass")
    or die "Couldn't connect to database: " . DBI->errstr;

    my $rowCount = 0;    
    my $filename = "Output012023.xlsx";
    my $workbook = Excel::Writer::XLSX->new( $filename );

    open(FH, "<", "Debt.csv" ) or die;
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#2 is to add a comment every time you figure something out:

  foreach(@conf){

    my $line = $conf[$i];
    # the reason for doing this weird thing
    my $t.   = "".$i;
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The comments will significantly reduce the cognitive load needed to comprehend the code by outsourcing all the reasons for this and that to brief human readable comments.