PERL_COPY_ON_WRITE has nothing to do with threads or process as far as I can see.

It is used by code within the pp_hot.c, regcomp.h and other places to avoid copying data until (and unless) it is written to.

An example with informative comment comes from pp_hot.c: 

#ifdef PERL_COPY_ON_WRITE
/* The match may make the string COW. If so, brilliant, because that's
   just saved us one malloc, copy and free - the regexp has donated
   the old buffer, and we malloc an entirely new one, rather than the
   regexp malloc()ing a buffer and copying our original, only for
   us to throw it away here during the substitution.  */
if (SvIsCOW(TARG)) {
    sv_force_normal_flags(TARG, SV_COW_DROP_PV);
} else
#endif

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In reply to Re: What is PERL_COPY_ON_WRITE Compile Option About by BrowserUk
in thread What is PERL_COPY_ON_WRITE Compile Option About by DeadPoet

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