Dave, I took the liberty to add your explanation almost verbatim to perlref. 
You may want to polish it later, but I think it's pretty important to put that 
info in. And your explanation and the example were pretty clear to me. I 
mainly stressed that it's the string form of eval (it doesn't apply to eval {} )

Also I hope that I put it in the right place in perlref, as it explains 
closures in two places.

Here is the patch:

--- pod/perlref.pod.orig	2003-11-28 00:58:48.000000000 -0800
+++ pod/perlref.pod	2003-11-28 01:06:17.000000000 -0800
@@ -195,6 +195,51 @@
  continue to work as they have always worked.  Closure is not something
  that most Perl programmers need trouble themselves about to begin with.

+Note that lexical variables used inside the string form of C<eval()> 
+might not create a closure if not references elsewhere in the
+subroutine. This is because only when the string is eval'ed at
+run-time, can perl see that a variable is references from within the
+the eval'ed string. At compile time it's not known whether there is a
+lexical variable inside the string or not. For example:
+
+    {
+	my $x = 1;
+	sub f1 { print "f1: x=$x\n" }
+	sub f2 { eval 'print "f2: x=$x\n"' }
+    }
+
+    f1;
+    f2;
+
+produces the following output:
+
+  f1: x=1
+  Use of uninitialized value in concatenation (.) or string at (eval 1) line 1.
+  f2: x=
+
+What happens here is that when f1 is compiled, the compiler notices
+that the sub f1 makes mention of the outer lexical $x, so f1 gets its
+own private reference to that variable. When f2 is compiled, it has no
+such mention of $x, so it doesn't also capture $x.
+
+After the { } block is exited, the interpreter discards the current
+(and only) instance of $x. When f1 is later called, it still has its
+private copy of $x, and so can print out its value. When the eval is
+compiled via f2, f2 hasn't got a private copy of $x, so the eval tries
+to grab the value of the 'real' $x, which is now undef.
+
+In the current development version of perl, you get a warning when
+this happens:
+
+  f1: x=1
+  Variable "$x" is not available at (eval 1) line 1.
+  Use of uninitialized value in concatenation (.) or string at (eval 1) line 1.
+  f2: x=
+
+Therefore be careful when wanting to create a closure and using the
+string form of eval.
+
+
  =item 5.

  References are often returned by special subroutines called constructors.




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