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

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

my @append = (
              {
               test => 1,
               key  => 3
              },
              {
               abc => 2,
               xyz => 5
              }
             );

@{$hoh{element1}}{map keys %{$_}, @append} = map values %{$_}, @append;

print Dumper \%hoh;