my $client = REST::Client->new();

$client->setHost("http://myhost.com");

$client->POST('/xx/yy/Submit',
              $params,
              {'Content-type' => 'application/json'}
             );

my $response = $client->responseContent();

{
    "Credential": {
            "Username": "String",
            "Password": "String"
    },
    "DataCoding": "Default",
    "Header": {
            "From": "String",
            "ValidityPeriod": 0
    },
    "Message": "String",
    "To": ["String"]
}

my %params = (Username => $username,
              Password => $password,
              From     => $from,
              ValidityPeriod => 5,
              Message  => "Test",
              To       => '90535xxxxxx',
           );

my $params = join("&",map("$_=$params{$_}",keys %params));

<Submit xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="SmsApi">
    <Credential>
            <Password>String</Password>
            <Username>String</Username>
    </Credential>
    <DataCoding>Default</DataCoding>
    <Header>
            <From>String</From>
            <ScheduledDeliveryTime>DateTime</ScheduledDeliveryTime>
            <ValidityPeriod>0</ValidityPeriod>
    </Header>
    <Message>String</Message>
    <To xmlns:d2p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
            <d2p1:string>String</d2p1:string>
    </To>
</Submit>