wbile( my $workitem = sourceOfWork() ) {
    sleep 1 while $Q->pending > MAXQ;
    $Q->enqueue( $workitem );
}

##</code><code>##

my $cond :shared;

...
wbile( my $workitem = sourceOfWork() ) {
    cond_wait( $cond );
    $Q->enqueue( $workitem );
}


## somewhere else ...
...
cond_signal( $cond ) if $Q->pending < MAXQ;
...