With Tk, you MUST create the thread before any Tk objects are created, because duplicate copies of them will get cross-linked in the threads, and cause weird errors. Here is the simplest solution. Create your threads first, and put them in a sleep loop. Then build your Tk frontend and control the threads thru shared variables. Also, do not attempt to directly modify a Tk widget from another thread. Use shared variables to signal changes, then let the main Tk thread adjust the widgets. See Tk-with-worker-threads and PerlTk on a thread.... You can also share filehandles between threads, here is a simple example (without Tk, but you could display the output in a text widget if desired)

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

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);

 $shash{'go'} = 1;

sleep 1;  # cheap hack to allow thread to setup

my $fileno = $shash{'fileno'};
open (my $fh, "<&=$fileno") or warn "$!\n";

while ( <$fh> ){ print "In main-> $_";  }

#wait for keypress to keep main thread alive
<>;

# control-c to exit
#####################################
sub work{
    $|++; 
    while(1){
       if($shash{'die'} == 1){ goto END }; 
      
       if ( $shash{'go'} == 1 ){

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