Try like this Re: threads causing memory leak (No leak!)(Win32::GUI DoOneEvent threads poll) , maybe organized like this

#!/usr/bin/perl --
use strict;
use warnings;
use threads;
use Thread::Queue;

## how $guithread and $mechthread  communicate
my $DataQueue = Thread::Queue->new();

my $guithread = threads->create(
    sub {
        require MyApp::GUI; ## MyApp/GUI.pm
        MyApp::GUI::go(@_); ## uses timer to check on queue
    },
    $DataQueue,
);

my $mechthread = threads->create(
    sub {
        require MyApp::Mech; ## MyApp/Mech.pm
        MyApp::Mech::go(@_);
    },
    $DataQueue,
);

$guithread->join; ## wait for gui to finish
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