To get XML of a specific format, work backwards. Start with xml exactly how you like it and see what type of data structure it makes:

#!/usr/bin/perl
use Data::Dumper;
use XML::Simple;

use strict;
use warnings;

my $data = do {local $/; <DATA>};
my $xml = XMLin($data);
print Dumper($xml);

__DATA__
<Parameters>
<ParameterGroup ID="Group1">
<Parameter key="Key1">Some Value</Parameter> 
<Parameter key="Key2">Some Value</Parameter> 
<Parameter key="Key3">Some Value</Parameter> 
</ParameterGroup>
<ParameterGroup ID="Group2">
<Parameter key="Key1">Some Value</Parameter> 
<Parameter key="Key2">Some Value</Parameter> 
<Parameter key="Key3">Some Value</Parameter> 
</ParameterGroup>
</Parameters>
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Doing this shows us our data structure. Now we just need to build it before translating to xml:

#!/usr/bin/perl

use XML::Simple;

use strict;
use warnings;

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',
);
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Outputs

<Parameters>
  <ParameterGroup ID="Group1">
    <Parameter key="Key1">Some Value</Parameter>
    <Parameter key="Key2">Some Value</Parameter>
    <Parameter key="Key3">Some Value</Parameter>
  </ParameterGroup>
  <ParameterGroup ID="Group1">
    <Parameter key="Key1">Some Value</Parameter>
    <Parameter key="Key2">Some Value</Parameter>
    <Parameter key="Key3">Some Value</Parameter>
  </ParameterGroup>
</Parameters>
[download]