You want to use Tk::fileevent. It is like IO::Select for Tk. This is untested, but should show you the idea. I'm not sure how your tie *STDOUT will work here. What fileevent does, is test for data in a filehandle, and if "readable", will read it and insert it into your Text widget. It is non-blocking, so your GUI will continue to run.

sub conn{
    tie *STDOUT, 'Tk::Text',$t;
    $|++;

    $sock = new IO::Socket::INET(PeerAddr => 'myhost',
                             PeerPort => '800',
                             Proto    => 'tcp');
                       
    die unless $sock;
   
#    while (defined($buff = <$sock>)){
#       print $buff;
#  }

$mw->fileevent($sock, 'readable', [\&fill_text_widget,$t]);
   
}


sub fill_text_widget {
          my($widget) = @_;
          $_ = <$sock>;
          $widget->insert('end', $_);
          $widget->yview('end');
}
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