You could try something recursive like this:

use warnings;
use strict;
use Data::Dump::Streamer;

my %hash1 = (
  foo => 'bar',
  baz => { quux => 'qwe',
           rty => 'uiop', },
  perl => ['monks', 'monastery', 'robes'],
);

my %hash2 = (
  wizardry => 'hocus-pocus',
  foo => 'f00',
  baz => { grail => 'arthur', },
  perl => ['yayy', 'yippee'],
  hash3 => {key1 => 'value1', key2 => 'value2',  },
);

merge (\%hash2, \%hash1);
Dump (\%hash1);

sub merge {
  my ($source, $target) = @_;
  
  for (keys %$source) {
    if ('ARRAY' eq ref $target->{$_}) {
      push @{$target->{$_}}, @{$source->{$_}};
    } elsif ('HASH' eq ref $target->{$_}) {
      merge ($source->{$_}, $target->{$_});
    } else {
      $target->{$_} = $source->{$_};
    }
  }
}
[download]

$HASH1 = {
           baz      => {
                         grail => 'arthur',
                         quux  => 'qwe',
                         rty   => 'uiop'
                       },
           foo      => 'f00',
           hash3    => {
                         key1 => 'value1',
                         key2 => 'value2'
                       },
           perl     => [
                         'monks',
                         'monastery',
                         'robes',
                         'yayy',
                         'yippee'
                       ],
           wizardry => 'hocus-pocus'
         };
[download]