Actually, simply calling length on a scalar with UTF8=1 adds the magic.
>perl -MDevel::Peek -e"Dump $_=chr(0x2660)x100; length $_; Dump $_"
SV = PV(0x2379ec) at 0x1845eec
REFCNT = 1
FLAGS = (POK,pPOK,UTF8)
PV = 0x184c5ec "..."\0 [UTF8 "..."]
CUR = 300
LEN = 304
SV = PVMG(0x18242cc) at 0x1845eec
REFCNT = 1
FLAGS = (SMG,POK,pPOK,UTF8)
IV = 0
NV = 0
PV = 0x184c5ec "..."\0 [UTF8 "..."]
CUR = 300
LEN = 304
MAGIC = 0x1824e64
MG_VIRTUAL = &PL_vtbl_utf8
MG_TYPE = PERL_MAGIC_utf8(w)
MG_LEN = 100 <---------- char length
It's a pity that actions such as chop, appending a UTF8=0 string, etc void the count instead of updating it.
Note that $_ eq '' doesn't add the magic, so not only is it faster, it uses less memory.
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