14 seconds overall, or 14 seconds just for the undef. When I try this on freebsd I find that it takes twice as long to create the hash than to undef it:

$ perl hash.pl ; perl hash.pl ; perl hash.pl
Create:  5.26445007324219
Destroy: 1.82375288009644
Create:  5.18459510803223
Destroy: 1.82551097869873
Create:  5.14320707321167
Destroy: 1.8236780166626
[download]

using a slightly different program:

$ cat hash.pl
#!/usr/bin/perl

use strict;
use warnings FATAL => 'all';
use Time::HiRes 'time';

my $t0 = time;
my %hash;
$hash{$_} = 1 for 1..3_000_000;
my $t1 = time;
undef %hash;
my $t2 = time;

print "Create:  ", $t1-$t0, "\nDestroy: ", $t2-$t1, "\n";
[download]

$ uname -a
FreeBSD XXX.XXX.XXX 7.0-RELEASE-p2 FreeBSD 7.0-RELEASE-p2 #0: Wed Jun 
+18 06:48:16 UTC 2008     root@amd64-builder.daemonology.net:/usr/obj/
+usr/src/sys/GENERIC  amd64
[download]

$ perl -v

This is perl, v5.8.8 built for amd64-freebsd
[download]

And I get very similar results on linux with perl 5.10.0 and 5.8.8