use strict;
use warnings;
use Data::Dumper;
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 %consolidated;

foreach (@servers) {
    push @{$consolidated{$_->{Host}}}, $_->{GroupName};
}

print Dumper(\%consolidated);