Your problem is that you are never reaching your sleep statement. Your loops is

while (1) {
    ....#check drive freespace
    my $low = @lowDrives > 0 ? join(' and ',lowDrives) : No';
    next if ($low eq 'No'); ## <<< PROBLEM HERE!
    ....
    sleep 2 *3600;
    ....
}
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Which means that if none of your drives are low, you do next, which goes back and checks again. You never reach the sleep.

sleep works fine, you're just never actually calling it.