Parallel::ForkManager can do what you want.

use Parallel::ForkManager;
my $max = 3;
my %cids = ();
my $forker = new Parallel::ForkManager($max);
while (<REQUEST>) {
    if (pressure() > 0) {
        $max = $forker->set_max_procs( $max + 3) ;
    }
    elsif {pressure() < 0) {
        $max = 1 + scalar( keys %cids);
        $forker->set_max_procs( $max);
    }
    if (my $pid = $forker->start()) {
        ++$cids{$pid};
        next;
    }
    act_on($_);
    $forker->finish();
}
$forker->wait_on_children();
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With adjustments and subs to fit your purpose.

After Compline,
Zaxo