How can I update the GUI then?

Here's one way:

#! perl -slw
use strict;
use threads;
use Thread::Queue;
use Tk::Listbox;

my $Q = new Thread::Queue;

## start a thread monitoring events
async {
    ## Simulate asynchronous events with random sleeps
    while( sleep 1+rand 3 ) {
        ## When an event happens, Q it to the GUI thread.
        $Q->enqueue( "Event " . time );
    }
}->detach;

my $mw = MainWindow->new;
my $lb = $mw->Listbox()->pack;

# 10 times per second
my $repeat = $mw->repeat( 100 => sub {
    ##check to see if there are any events in the queue
    while( $Q->pending ) {
        ## if there are, grab them
        my $event = $Q->dequeue;
        ## adde them to the end of the listbox
        $lb->insert( 'end', $event );
        ## and make sure the latest is visible.
        $lb->see( 'end' );
    }
});

$mw->MainLoop;
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