I'm not a big fan of "validating inheritance" (especially in Perl where you can make perfectly functional objects [that simply implement all of the needed methods] that fail the 'isa' test) so rather than using UNIVERSAL::isa(), I'd probably either do something like have all error objects define an isError() method and add

    sub UNIVERSAL::isError { 0 }
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or have all error objects define a getError() method and test with $data->can("getError").

Better still, have your error objects overload boolean context so that they return "false" and just do

    if(  $data  ) {
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which starts heading toward what I've been wanting to write for years now: Error objects that fake all possible method calls to return themselves when called in a "object/method" context. They overload boolean context such that they return "false" and note that they have been "tested". If they get used in any other context or if they get destroyed without having been "tested", then they die with a full error message and stack trace.

Then you don't have to worry about checking for errors. If you don't check for success, then your code automatically dies on failure. If you do check for success but get it wrong, your code again dies on failures. If you do check for success and get it right, nothing dies. And you don't have to check right away so you can write:

    # Dies if any method fails:
    $value= $Registry->SetOption(Delimiter=>"/")
        ->OpenKey("This")->GetValue("That");
    # Doesn't die, no matter which method fails:
    if(  ! $Registry->OpenKey("Foo")->GetValue("Bar")  ) {
        # Create missing bits here
    }
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(: [updated]

                - tye