$ cat pm_hash_clear_vs_undef_1214851.pl
use strict;
use warnings;
use Benchmark ':all';
use Hash::Util 'bucket_stats';

my @some_keys = ('A' .. 'Z', 'a' .. 'z', '0' .. '9');

print "Checking container differences between delete and clear:\n";
my %gh;
print "Hash size (initial): ", old_hash_stats(\%gh), "\n";
fill_hash(\%gh);
print "Hash size (after fill): ", old_hash_stats(\%gh), "\n";
%gh=();
print "Hash size (after clear): ", old_hash_stats(\%gh), "\n";
undef %gh;
print "Hash size (after delete): ", old_hash_stats(\%gh), "\n";
print "\n";

sub old_hash_stats {
    my $hr = shift;
    my @hash_stats = bucket_stats($hr);
    return "$hash_stats[0]/$hash_stats[1]";
}

sub fill_hash {
    my $hr = shift;
    @{$hr}{@some_keys} = (0) x @some_keys;
}

cmpthese(500000, {

    'overwrite' => sub {
        my %h;
        fill_hash(\%h);
        fill_hash(\%h);
    },

    'delete' => sub {
        my %h;
        fill_hash(\%h);
        undef %h;
        fill_hash(\%h);
    },

    'reuse' => sub {
        my %h;
        fill_hash(\%h);
        %h = ();
        fill_hash(\%h);
    },
});