What am I doing wrong?

• You are trying to use a Thread::Queue as a semapahore.

  Which is not its purpose.

• You are using the peek() method of Thread::Queue.

  This method, along with insert() and extract() should never have been added to a queue module.

  They create far more problems than they solve.

• The specific issue with your code is this:

  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 y
  +our threads with the queue empty/
   my $mem_monitor_thread = threads->create( \&monitor_mem )
  [download]

  When those threads run:

  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;
   }
  [download]

  The queue is empty so they go straight passed the loop and out the other end.

If all you want is a way to tell your threads to stop, use a single shared variable:

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;
[download]

Though quite what the purpose of your cpu/mem monitoring is, I do not understand, because you are only going to get the last report returned, which is utterly meaningless as a measure of anything.