package MyConfig;
...
our %Config = (
    ...
);

sub getConfig {
    my( $key ) = @_;
    our $Host ||= hostname();
    our %HostConfig;
    return $HostConfig{$key} //= _getConfigForHost( $Host, $key );
}

##</code><code>##

sub getConfig {
    my( $key, $host ) = @_;
    $host ||= hostname();
    our %HostConfig;
    return $HostConfig{$key,$host} //= _getConfigForHost( $host, $key );
}

##</code><code>##

$MyConfig::Host = $test_host;

##</code><code>##

for my $host ( ... ) {
    $MyConfig::Host = $host;
    %MyConfig::HostConfig = ();
    ...
}