Great catch! But here's a wrinkle. Other common uses of STDERR redirected to string, namely print, do not require the string variable to be initialized. For example:

use strict;
use warnings;

my $sOut;
close(STDERR);
open(STDERR, ">", \$sOut)
  or die "Can't redirect STDERR to \\\$sOut";
print STDERR "Hello World!\n";
print $sOut;
[download]

happily outputs "Hello World" without generating a warning (at least on my system). Somehow, I don't think the Devel::Peek::Dump behavior qualifies as Inconsistent for the sake of convenience :-)

Best, beth