Cause: The internal newSVsv() routine was called to duplicate a scalar that had previously been marked as free.

What does this error actually mean?

Google turned up:

Cause: The internal newSVsv() routine was called to duplicate a scalar that had previously been marked as free.

But what does "marked free" mean? Is that a euphemism for "who's reference count has dropped to 0"?

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to semi-panic: attempt to dup freed string? by BrowserUk

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