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
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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.