I just noticed the footnote in the OP. Sounds like you have C/XS code that modifies the string buffer which uses SvPV, SvPVbyte or SvPVutf8 instead of SvPV_force, SvPVbyte_force or SvPVutf8_force. Bad!

In reply to Re^2: Concatenation with empty string -- good enough to force physical copy? by ikegami
in thread Concatenation with empty string -- good enough to force physical copy? by vr

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