use 5.026;
use warnings;

use Data::Dumper;

my @servers = (
   {
       Host      => q{server04},
       GroupName => q{serversB},
   },
   {
       Host      => q{server02},
       GroupName => q{serversC},
   },
   {
       Host      => q{server04},
       GroupName => q{serversA},
   },
   {
       Host      => q{server02},
       GroupName => q{serversD},
   },
   {
       Host      => q{server02},
       GroupName => q{serversB},
   },
   {
       Host      => q{server02},
       GroupName => q{serversA},
   },
   {
       Host      => q{server03},
       GroupName => q{serversC},
   },
   {
       Host      => q{server04},
       GroupName => q{serversC},
   },
   );

my %inGroups;
foreach my $rhServer ( @servers )
{
    push @{ $inGroups{ $rhServer->{ Host } } }, $rhServer->{ GroupName };
}

say Data::Dumper
   ->new( [ \ %inGroups ], [ qw{ *inGroups } ] )
   ->Sortkeys( 1 )
   ->Dumpxs();

say qq{Server $_ is in groups },
   join q{, }, sort @{ $inGroups{ $_ } }
   for sort keys %inGroups;

##</code><code>##

%inGroups = (
              'server02' => [
                              'serversC',
                              'serversD',
                              'serversB',
                              'serversA'
                            ],
              'server03' => [
                              'serversC'
                            ],
              'server04' => [
                              'serversB',
                              'serversA',
                              'serversC'
                            ]
            );

Server server02 is in groups serversA, serversB, serversC, serversD
Server server03 is in groups serversC
Server server04 is in groups serversA, serversB, serversC