my $xml = { ParameterGroup => [
    {
        ID => 'Group1',
        Parameter => {
            Key1 => {content => 'Some Value'},
            Key2 => {content => 'Some Value'},
            Key3 => {content => 'Some Value'},
        },
    },
    {
        ID => 'Group1',
        Parameter => {
            Key1 => {content => 'Some Value'},
            Key2 => {content => 'Some Value'},
            Key3 => {content => 'Some Value'},
        },
    },
]};

print XMLout($xml,
    RootName => 'Parameters',
    KeyAttr => 'key',
);