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;