# let's suppose the list of services is the primary HoH structure:
#  -- top-level hash is keyed by service name
#  -- next-level hash is keyed by host name
#  -- $service{service_name}{host_name} = "command line";

my %service = (
    servicea => { host1 => "/usr/local/bin/servicea -foo",
                  host2 => "/usr/bin/servicea -bar",
                  ... },
    serviceb => { host2 => "/usr/bin/serviceb -baz",
                  host3 => "/usr/local/bin/serviceb -boo"
                  ... },
    ...
);

# now populate %host HoH to store the same information, but
# with host names as top-level keys and service names as sub-keys:

my %host;
for my $srvc ( keys %service ) {
    for my $hst ( keys %{$service{$srvc}} ) {
        $host{$hst}{$srvc} = $service{$srvc}{$hst};
    }
}

# to set up an option menu of services for a particular host,
# use the %host hash:

my $hostname = `hostname`;
chomp $hostname;

my @available_services = sort keys %{$host{$hostname}};

...

# to show which hosts are available to run a given service,
# use the %service hash:

my $servicename = ...; # assuming there's some service of interest

my @available_hosts = sort keys %{$service{$servicename}};