Thanks for the replies, the open is working properly on the background. I'm able to keep working while the $cmd is running.

The $reg_out is output to a text gadget. If you think the open cmd is the issue, how should I fix it? I want to run a CMD on the background, grep the output to be paste on a text gadget on the main process gui

This is the code of the fileevent, I'm closing the descriptor once the program is done

 sub fill_text_widget {
my $block = shift;
my $numb = shift;
my $widget = $Reg_r{$block}{$numb}{'Txt'};
my $out_rep = $Reg_r{$block}{$numb}{'OutP'};
if (eof($out_rep))
{
    $widget->fileevent($out_rep,"readable", undef);
    close $Reg_r{$block}{$numb}{'OutP'};
    print "Got EOF\n" if ($DBG);
    $Reg_r{$block}{$numb}{'IsRun'} =0;
    &AddReRun($block, $numb);
    return;
}
$_ = <$out_rep>;
$widget->insert('end', $_);
$widget->yview('end');
}
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