I would just mark the process done by zeroing the hash value. Then at strategic points in the parent-side code I would "clean up" the list.

Here is a diff to your code that implements that change. Mind you though, it is completely untested so there may still be gaping holes.

--- Ext/Fork.pm.00    2009-12-19 06:00:59.000000000 -0800
+++ Ext/Fork.pm    2009-12-20 15:16:22.000000000 -0800
@@ -109,6 +109,7 @@
     }
 
     while(1){
+        rfork_cleanup();
         if($Ext::Fork::POOL->{children} < $Ext::Fork::POOL->{max_chil
+dren}){
             last;
         }
@@ -145,6 +146,7 @@
 
     if($Ext::Fork::POOL->{has_children} && !$Ext::Fork::POOL->{nonblo
+cking}){
         while(1){
+            rfork_cleanup();
             last if !$Ext::Fork::POOL->{children};
             if($Ext::Fork::has_thr){
                 rfork_usleep(500);
@@ -195,8 +197,7 @@
 
 sub _sigchld {
     while((my $p = waitpid(-1, WNOHANG)) > 0){ 
-        delete $Ext::Fork::POOL->{cidlist}->{$p};
-        $Ext::Fork::POOL->{children} -- if $Ext::Fork::POOL->{childre
+n};        
+        $Ext::Fork::POOL->{cidlist}->{$p} = 0;    # mark process as d
+one
     }
 
     # self reference
@@ -243,6 +244,7 @@
     return 1 if $Ext::Fork::POOL->{nonblocking};
 
     while(1){
+        rfork_cleanup();
         last if !$Ext::Fork::POOL->{children};
 
         if($Ext::Fork::has_thr){
@@ -262,6 +264,7 @@
 =cut
 
 sub rfork_active_children {
+    rfork_cleanup();
     return ($Ext::Fork::POOL->{children} ? $Ext::Fork::POOL->{childre
+n} : 0);
 }
 
@@ -391,6 +394,7 @@
 
 sub rfork_kill_children {
     my $sig = $_[0];
+    rfork_cleanup();
     if(!$sig){
         $sig = 'TERM';
     }
@@ -410,6 +414,7 @@
 
 sub rfork_list_children {
     my ($use_hash) = @_;
+    rfork_cleanup();
 
     if(!$Ext::Fork::POOL->{cidlist}){
         return;
@@ -432,6 +437,7 @@
 
 sub rfork_child_dob {
     my $pid = $_[0];
+    rfork_cleanup();
     if($Ext::Fork::POOL->{cidlist}->{$pid}){
         return $Ext::Fork::POOL->{cidlist}->{$pid};
     } else {
@@ -439,4 +445,21 @@
     }
 }
 
+=head2 rfork_cleanup()
+
+Perform delayed list cleanup.
+
+=cut
+
+sub rfork_cleanup {
+    my @deadpids =
+        grep { $Ext::Fork::POOL->{cidlist}->{$_} == 0 }
+        keys %{ $Ext::Fork::POOL->{cidlist} };
+    foreach (@deadpids)
+    {
+        delete $Ext::Fork::POOL->{cidlist}->{$_};
+        $Ext::Fork::POOL->{children} -- if $Ext::Fork::POOL->{childre
+n};        
+    }
+}
+
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