but then I want to add an end => 10 to the same anonymous hash without replacing what's already there

use warnings;
use strict;
use Data::Dumper;

my %hoh;
$hoh{element1} = {start => 1,
                  middle => 5};

print Dumper \%hoh;

 $hoh{element1}{end}=10;  # add to hash

print Dumper \%hoh;
[download]

OUTPUT:

$VAR1 = {
          'element1' => {
                          'middle' => 5,
                          'end' => 10,
                          'start' => 1
                        }
        };
[download]

For more information, please check the following: perldsc, perlref, perllol, perldata, perlobj