Currently the only problem I have with it is that it doesn't croak if you forget the Parent or Child blocks, but I think that's pretty minor.

Easy enough:

--- ForkBlock.pm~    Wed Dec 24 20:18:01 2008
+++ ForkBlock.pm    Wed Dec 24 21:02:50 2008
@@ -31,6 +31,9 @@
     my $parent=$params{"parent"};
     my $child =$params{"child"} ;
     my $error =$params{"error"} ;
+
+    croak "No Child defined" unless $child;
+    croak "No Parent defined" unless $parent;
     
     local $!;
[download]

Although one could make the case that it ought to fall-through without a Parent. You can achieve that with DumbFork { Parent {return} ...}

This patch exposes the child PID in a non-collisiony way, as a parameter to the parent; you could also copy $ForkBlock::CHILD_PID immediately after fork.

--- ForkBlock.pm~    Wed Dec 24 21:02:50 2008
+++ ForkBlock.pm    Wed Dec 24 21:14:35 2008
@@ -40,7 +40,7 @@
     FORK_IT: {
         if($CHILD_PID=fork()) {
             #i'm the parent
-            &$parent;
+            &$parent($CHILD_PID);
       }
         elsif(defined($CHILD_PID)) {
             #i'm the child
[download]

Also note that the prototype permits this alternate invocation:

Fork {
    Parent \&manager,
    Child  \&worker
};
[download]

And with this patch:

--- ForkBlock.pm~    Wed Dec 24 21:14:35 2008
+++ ForkBlock.pm    Thu Dec 25 02:08:18 2008
@@ -27,10 +27,10 @@
 #private--implements the actual forking
 sub phork {
     my %params=%{shift()};
-    
-    my $parent=$params{"parent"};
-    my $child =$params{"child"} ;
-    my $error =$params{"error"} ;
+
+    my $parent=$params{parent}|| $params{Parent};
+    my $child =$params{child} || $params{Child};
+    my $error =$params{error} || $params{Error};
 
     croak "No Child defined" unless $child;
     croak "No Parent defined" unless $parent;
[download]

You can also do:

Fork {
    Parent=>sub{ ... }, #The comma's important here.
    Child =>sub{ ... }
};
[download]

The distinction between DumbFork and Fork seems backwards/unnecessary. What if the exit has a condition of if $CHILD_PID == 0 added? Another interesting change might be an optional/default Reaper that's invoked before the phork...