timethese 10000, {
    this => ' ...... ',
    that => ' ...... '
};

##</code><code>##

timethese 10000, {
    this => sub { ..... },
    that => sub { ..... }
}

##</code><code>##

use strict;
use warnings;
use Benchmark;

my (%h_delete, %h_list, %h_undef);
%h_delete = %h_list = %h_undef = (1 .. 50000);

delete @h_delete{keys %h_delete};
%h_list = ();
undef %h_undef;

timethese 10000, {
    delete  => sub{ 1 for keys %h_delete},
    list    => sub{ 1 for keys %h_list},
    undef   => sub{ 1 for keys %h_undef},
    };