your best bet to handle it, would be to pass the fileno of $fr_child back to the main thread

Who's gonna read from that handle? The OP wants the program to run asynchronously with the Tk window, such that output from the program gets added to the Tk window as it is produced.

Who's gonna read from that handle?

On linux, you would setup a fileevent on that fileno, and as readable data appears on the filehandle, it would be inserted into the text box. However, on win32, fileevent won't work reliably on pipes, so you will need a timer to repeatedly read the fileno in the main script.

Here is a simple example of passing the fileno back from a thread.(works on linux)

#!/usr/bin/perl
use warnings;
use strict;
use threads;
use threads::shared;
use Tk;

my %shash;
#share(%shash); #will work only for first level keys
my %hash;

 share ($shash{'go'});
 share ($shash{'fileno'});
 share ($shash{'pid'});
 share ($shash{'die'});
 
 $shash{'go'} = 0;
 $shash{'fileno'} = -1;
 $shash{'pid'} = -1;
 $shash{'die'} = 0;

 $hash{'thread'} = threads->new(\&work);

my $mw = MainWindow->new(-background => 'gray50');

my $text = $mw->Scrolled('Text')->pack();


my $startb = $mw->Button( -text => 'Start',
                -command=>sub{
                     $shash{'go'} = 1;
             $mw->after(100); #give pipe chance to startup
             my $fileno = $shash{'fileno'};
                     print "fileno_m $fileno\n";
          open (my $fh, "<&=$fileno") or warn "$!\n";

 # filevent works on linux but may not work on win32, 
 # but you can use a timer instead as shown below
 $mw->fileevent(\*$fh, 'readable', );             
             while(<$fh>){
            $text->insert('end',$_);    
                $text->see('end');                  
                $mw->update;
           }

# on Win32 (untested by me) you will need 
# a timer instead of fileevent

#   my $repeater; 
#        $repeater = $mw->repeat(10,
#          sub {
#              my $bytes = sysread( "<&=$fileno", my $buf, 8192);
#          $text->insert('end',$buf);    
#                     $text->see('end');         
#          if( $shash{'go'} == 0 ){ $repeater->cancel }
#               }
#             );             
        
             }
                 )->pack();

my $stopb = $mw->Button( -text => 'Stop/Exit',
                -command=>sub{
                 $shash{'die'} = 1;
                 kill 9,$shash{'pid'};
                 $hash{'thread'}->join;         
                 exit;
              },
                 )->pack();


MainLoop;
#####################################

sub work{
    $|++; 
    while(1){
       if($shash{'die'} == 1){ return }; 
      
       if ( $shash{'go'} == 1 ){

        my $pid = open(FH, "top -b |" ) or warn "$!\n";
        my $fileno = fileno(FH);
        print "fileno_t->$fileno\n";
        $shash{'fileno'} = $fileno;
        $shash{'pid'} = $pid;
       
    $shash{'go'} = 0; #turn off self before returning      
       }else
         { select(undef,undef,undef,.1) } #short sleep
    }

}
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I'm not really a human, but I play one on earth Remember How Lucky You Are

so you will need a timer to repeatedly read the fileno in the main script.

That would block. And as far as I know, it's impossible to detect whether the pipe has data waiting. (select won't work.) Perl imposes a unix view of the world, and that's problematic where the Windows and unix approach of performing a task are fundamentally different.

But polling is the indeed the answer. You could poll a queue that's populated by a worker thread that reads the child's output.