use warnings;
use strict;

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

my %hosts;
for my $serv (@servers) {
    $hosts{ $serv->{Host} }{ $serv->{GroupName} }++;
}

for my $host (sort keys %hosts) {
    print "$host is in Groups ",
        join(',', sort keys %{$hosts{$host}}), "\n";
}
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