Thank you for your help!

finally I solved it with threads:

#!/usr/bin/perl
use threads;
use threads::shared;
require Thread::Queue;

$Q = new Thread::Queue;

share $Script;
share $Terminate;
share $start_work;

$Terminate =0;
$start_work = 0;

threads->new(\&worker)->detach;

use Tk;
require Tk::ROText;
$top = MainWindow->new;
$text = $top->Scrolled('ROText');
    $text->pack(qw/-expand 1 -fill both/);

$b_startdir = $top->Button(
    -text    => "Start command",
    -width   => 12,
    -command => sub {
        $Script = "ping example.org";
        $start_work = 1;
        }
)->pack( -side => 'right' );


my $repeat;
$repeat = $top->repeat( 300 => sub {
    eval {
    while ($Q->pending) {
      my $buffer = $Q->dequeue;
      return unless $buffer;
      
      $text->insert('end', $buffer);
      $text->see('end');
    }
    };
    if ($@) {
      warn "$@";
      $repeat->cancel;
    }
} );

MainLoop;
$Terminate = 1;

sub worker {
  while(!$Terminate){
    if ($Script && $start_work) {
      open PROC, "$Script 2>&1 |" or die "ERR: unable to start $Script
+\n";
      while (sysread(PROC, my $buffer, 1024) > 0) {
        $Q->enqueue($buffer);
        if (!$start_work) {last;}
      }
      close PROC;
      $Script = undef;
      $start_work = 0;
    } else {
      sleep 1;
    }
  }
}
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