How to search for a unique Object using a CustomField ?

I'll preface this with: this is my 1st time with RT

I am migrating our organization to RT based on it's stated capabilities and extensibility, as well as it's support for assets

I've been told to maintain our Ticket history so I'm writing a [standalone] perl importing routine that will load RT with data from the old system

I'm writing it also as a sync-ing routine because it's going to take a while to get everything running and configured right - I expect to run it quite a few times

I have decided to tag each major object (user, ticket, asset) in RT with the original systems' object id in a custom field

So given an object type and an id from the old system I need to find that object in RT

I've been reading code and trying many different ways to accomplish this -- to no avail

What is best practice for finding an object within RT given the value 0f an expected CF ?

thanks
al;


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