*sigh* I'm sorry. No, that's not at all the case, and I agree that I probably mis-used the term 'crash'. I was trying very hard to remember not to say "die" to mean abnormal termination, because the behaviour wasn't a due to the die() command at all.

To put that more clearly, calling substr in a lvalue context, with parameters that lie entirely outside the target string, causes a fatal-but-silent termination of the Perl process.

Yes, that's exactly what I get! Again, my use of the term "crash" was probably misleading. I apologize: it seems we've been talking at cross-purposes for a while now. :-(

So, are we agreeing; disagreeing; agreeing to disagree or disagreeing to agree?

Well, I think we get the same output, and I think we both agree that it's odd. That counts as agreeing to me. :-)
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Ytrew