my $q = Thread::Queue->new; ## create a new **EMPTY** queue

 my $done : shared;
 $done = 0;

 my $wget_thread = threads->create( \&wget_sub );

 my $cpu_monitor_thread = threads->create( \&monitor_cpu ); ## Start your threads with the queue empty/
 my $mem_monitor_thread = threads->create( \&monitor_mem )

##</code><code>##

sub monitor_cpu {

     my $cap;
     while ( my $val = $q->peek(0) ) { #this does not get evaluated to true, ever
        $cap .= `bash cpu.sh; sleep 2`;
     }
     return $cap;
 }

##</code><code>##


my $done :shared = 0;
...
sub monitor_cpu {
    my $cap;
    until( $done ) { 
       $cap .= `bash cpu.sh; sleep 2`;
    }
    return $cap;
}

...

my $mon = threads->new( \&monitor_cpu );

... do other stuff

## To stop the queue(s)
$done = 1;
$mon->join;