I modified your script a little bit:

use strict;
use warnings;
use Time::HiRes qw(time);

# Create a large hash
my %hash;
my $count = 3_000_000;
$hash{$count} = rand(10) while $count--;

# Undefine it
print "Undefining hash.\n";
my $before = time;
undef %hash;
printf "Done in %.03f seconds\n", time - $before;
[download]

On my linux box it's about 1.5s for both perl 5.8.8 and perl 5.10.0.

I suspect that might be due to a different malloc in windows and linux, so my next step will be do compile a perl with a different malloc, and try again.

Update: A perl configured with -Uusemymalloc took 1.4s, so no real difference.