Howzabout this?:

my $abort = 0;

MainLoop;

sub startLoop {
    while (1) {
        sleep (2);
        print STDERR "startLoop: still sleeping\n";
        $mw -> update();
        last if $abort;
    }
}

sub interruptLoop {
    #something to interrupt startLoop and return to MainLoop?
    print STDERR "I've been pressed\n";
    $abort = 1;
}
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