The only comment is that it's not a HoA, it's a HoH.

The datastructure will look like this:

%HoH = (
    SA => {
        0009 => 1
    }
);
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It's done that way all the time. Your second method: $HoA{$robc,$telco} will work but not in the same way (ie, it's not a hash of hashes anymore). IMHO, it's probably better to use a HoH instead of a hash with some multi-part key.