Although it's not explicitly explained that I can find, my guess would be that under the hood it needs an actual not-undef SvPV* into which the data is written. Otherwise it's going to be trying to append things to the single global undefined value &PV_sv_undef which is what throws the error.

In reply to Re^3: stderr, local block and redirection by Fletch
in thread stderr, local block and redirection by jfroebe

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