Given the show of legitimate imports here, I think what's been going can be reasonable pieced together.
A small number of major AZA zoos seem to have imported Perentie. They have bred, I remember Omaha having a breeding success a few years ago. These animals then made their way into privately-owned specialist reptile facilities. These places are owned by big, trusted names in the herp keeping community and frequently end up with AZA surplus.
In the future, animals bred at these facilities may make their way out more widely into the captive trade, this has happened before to species with similar stories in the US, such as Komodo Dragon.