use XML::RPC;
use MIME::Base64;

my $xmlrpc = XML::RPC->new('https://user:pass@some.server.com/trac/project/xmlrpc');

my $result = $xmlrpc->call('ticket.putAttachment',
                           2,
                           'Filename',
                           'Some Description',
                           encode_base64('data, data - it is always the data')
                          );



Dumpering $result gives:

$VAR1 = {
          'faultString' => '\'\'str\' object has no attribute \'data\'\' while executing \'ticket.putAttachment()\'',
          'faultCode' => '2'
        };