While googling, I found this discussion of the XML RPC Plugin for Trac. I'm not sure how/whether this plugin is the one you use, but it lists the following Python code as sample code:

import xmlrpclib

server = xmlrpclib.ServerProxy("http://athomas:password@localhost:8080
+/trunk/login/xmlrpc")

server.wiki.putAttachment('WikiStart/t.py', xmlrpclib.Binary(open('t.p
+y').read()))
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So, likely, you can use that Python code to get a sample of what you have to send using XML::RPC. It seems to send a different number of paramters than your scample does, though, so maybe it's the wrong end in the tree.

From my cursory look at XML::RPC, your approach of simply sending the base64-encoded data should be OK, but maybe you need to replace it by:

my $result = $xmlrpc->call('ticket.putAttachment',
                           2,
                           'Filename',
                           'Some Description',
                           sub { +{base64 => encode_base64('data, data
+ - it is always the data')} },
                          );
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... which is what the "Custom Types" section at the end of the documentation suggests.