FWIW here are my results on MacOSX 10.5 PPC using MacPort's perl 5.8.8:

              s/iter    With undef Without undef
With undef      22.5            --           -9%
Without undef   20.6           10%            --
[download]

And with the system perl 5.8.8:

              s/iter    With undef Without undef
With undef      22.5            --           -8%
Without undef   20.7            9%            --
[download]

So on this platform the undef time is certainly measurable, but exactly an order less than the creation time.

Here is the code I used for the benchmark comparisons:

use strict;
use warnings;

use Benchmark qw(:all);

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

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

    # Undefine it
    undef %hash;
}

cmpthese(20, {
    'Without undef' => \&without_undef,
    'With undef' => \&with_undef,
});
[download]