sv_setsv(sv, &PL_sv_undef);

There are 16 instances of the idiom setsv(sv,
&PL_sv_undef) in the core:

gv.c:                       sv_setsv(*init,
&PL_sv_undef);
mg.c:   sv_setsv(sv, &PL_sv_undef);
mg.c:       sv_setsv(sv,&PL_sv_undef);
mg.c:   sv_setsv(sv,&PL_sv_undef);
mg.c:   sv_setsv(sv,&PL_sv_undef);
mg.c:   sv_setsv(sv,&PL_sv_undef);
mg.c:   sv_setsv(sv,&PL_sv_undef);
mg.c:                   sv_setsv(sv,&PL_sv_undef);
mg.c:    sv_setsv(sv, targ ? targ : &PL_sv_undef);
pp_hot.c:               sv_setsv(sv, &PL_sv_undef);
regcomp.c:        sv_setsv(sv,&PL_sv_undef);
regcomp.c:        sv_setsv(sv,&PL_sv_undef);
regcomp.c:        sv_setsv(sv,&PL_sv_undef);
sv.c:   sv_setsv(rv, &PL_sv_undef);
util.c:sv_setsv(sv, &PL_sv_undef); 
util.c:     sv_setsv(sv, &PL_sv_undef);


Is there any difference between

    setsv(sv, &PL_sv_undef);

and

    SvOK_off(sv);
    SvGETMAGIC(sv);

aside from the latter should be faster, as
Perl_sv_setsv_flags() is a stonking
great function.

Nicholas Clark