You forgot that the percentage for the progress bar must be a integer between 0 and 100. So using the following code works fine :

...
my $count = 0;
my $total = @data;
for (@data)
{
     print STDOUT "$_\n";
     $percent_done = $count * 100 / $total;
     $count++;
     $mw->update;
}
MainLoop();
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