Your syntax for calling XMLout is wrong. It's just print XMLout($xml);.

The reason it's printing wrong is that your structure is wrong. You have:

'phonenumbers' => [   # <- Repeats phonennumbers
   {
      'phonenumber' => {
         'number' => '9163723752'
      }
   },
   {
      'phonenumber' => {
         'number' => '9166413900'
      }
   }
]
[download]

You need:

'phonenumbers' => [        # <- Repeats phonennumbers
   {
      'phonenumber' => [   # <- Repeats phonennumber
         {
            'number' => '9163723752'
         },
         {
            'number' => '9166413900'
         }
      ]
   }
]
[download]

For example:

>type !.xml
<campaign action="0" menuid="10118">
  <phonenumbers>
    <phonenumber number="9163723752" />
    <phonenumber number="9166413900" />
  </phonenumbers>
</campaign>

>type !.pl
use XML::Simple qw( XMLin XMLout );

# It didn't think "!.xml" was a file name until I added "./".
# Options were lowercase in older versions (like the one I have.

my $xml = XMLin('./!.xml', keeproot => 1, forcearray => 1);
print XMLout($xml, keeproot => 1);

>perl !.pl
<campaign menuid="10118" action="0">
  <phonenumbers>
    <phonenumber number="9163723752" />
    <phonenumber number="9166413900" />
  </phonenumbers>
</campaign>
[download]

Update: Sorry, I was familiar with the OO calling syntax. My initial paragraph was wrong, but the rest still applies.