That works perfectly! And significantly reduces the amount of code!

FWIW, Here is the new code.

use warnings;
use strict;
use Tk;
use Tk::Table;
use Data::Dumper;

my $buttonsPerRow = 5;
$buttonsPerRow -= 1; 
my $row = 0;
my $col = 1;
my $buttonNum = 0;
my @buttons;

my $mw = tkinit();
my $menuFrame = $mw->Frame()->pack(-expand=>1,-fill=>'both');
my $buttonFrame = $mw->Frame()->pack(-expand=>1,-fill=>'both');

my $table = $buttonFrame->Table(
        -columns => 5,
        -scrollbars => 'se',
        -fixedrows => 1,
    )->pack(-expand=>1,-fill=>'both');

my $addButton = $menuFrame->Button(
        -text            => 'Add Button',
        -command     => sub{
            $buttons[$buttonNum] = $table->Button();
            print "Button $buttonNum->$row:$col\n";
            $buttons[$buttonNum]{'row'} = $row;
            $buttons[$buttonNum]{'col'} = $col;
            $table->put($row,$col,$buttons[$buttonNum]);
            configureButton($buttonNum);
            $col>$buttonsPerRow?$row++:undef;
            $col>$buttonsPerRow?$col=1:$col++;
            $buttonNum++;
        },
    )->pack(-side=>'top');

################################
#DragNDrop
my $isDragging = 0; #boolean - left mouse button pressed and dragging
my $selectedButton = 0;
$mw->bind('<Motion>', \&motion);

################################

$mw->MainLoop();

sub configureButton{
    #configure the new button
    my $btnNum = shift;
    $buttons[$btnNum]->bind('<Enter>', sub{
                $selectedButton = $btnNum;
            }
        );
    $buttons[$btnNum]->configure(
        -text => "Button $btnNum",
        -command => sub{
            print "Button $btnNum pressed\n";
        }
    );
    $buttons[$btnNum]->bind('<ButtonPress-1>', \&buttonPress);
    $buttons[$btnNum]->bind('<ButtonRelease-1>', \&buttonRelease);
}

sub motion {
    #what to do when the mouse is moving
    my($widget) = @_;
    my $e = $widget->XEvent;
    if ($isDragging){
        findCell($e->X,$e->Y);
    }
    return unless $isDragging;
}

sub buttonPress {
    $isDragging = 1;
}

sub buttonRelease {
    #what to do when the left mouse button is released
    $isDragging = 0;
}

sub findCell{      
    my $tmpX = shift;
    my $tmpY = shift;
    my ( $row, $col ) = $table->Posn($table->containing($tmpX,$tmpY));
  print "$tmpX:$tmpY: Row $row Col $col\n";
}
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