You are like the Sourcerer's Apprentice, :-) Threads by themselves don't magically make system calls non-blocking. You need to read deeper, and see that the system call needs to be run in a thread. This is called "blocking the Tk eventloop". See the Tk Event Loop

You might run your non-blocking code like this simple thread example. See, your system call is in the thread.

That is about as simple as I can explain it, of course, real code is usually a bit more complicated.

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

my $thr = threads->new(\&sub1);

print "thread running now\n";

$thr->join(); # wait for trivial thread to finish
                   # and return

sub sub1{
  system("documentation.doc");
  print "Done in thread\n";
}
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