$ perl -MDevel::Peek -e "\$_='qwerty';Dump(\$_);s/[rt]/Dump(\$_);/ge;print Dump(\$_)"
SV = PV(0x801838) at 0x82fde0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x201fe0 "qwerty"\0
  CUR = 6
  LEN = 8
SV = PVMG(0x823514) at 0x82fde0
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x201fe0 "qwerty"\0
  CUR = 6
  LEN = 8
  MAGIC = 0x2018d0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = PERL_MAGIC_regex_global(g)
    MG_LEN = 3
SV = PVMG(0x823514) at 0x82fde0
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x201fe0 "qwerty"\0
  CUR = 6
  LEN = 8
  MAGIC = 0x2018d0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = PERL_MAGIC_regex_global(g)
    MG_LEN = 4
SV = PVMG(0x823514) at 0x82fde0
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x2010d0 "qwey"\0
  CUR = 4
  LEN = 8
  MAGIC = 0x2018d0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = PERL_MAGIC_regex_global(g)
    MG_LEN = -1


##</code><code>##

$data =~s/([^\n]{16})/pos($data)=0;ch($1)/ge;

##</code><code>##

# perl -e "\$_='qwerty';s/r/print pos();/e;"
3

BUT

# perl -e "$_='qwerty';s/r/print pos();/e;"
syntax error at -e line 1, near ";="
Execution of -e aborted due to compilation errors.

# uname -v
Darwin Kernel Version 10.4.0: Fri Apr 23 18:28:53 PDT 2010; root:xnu-1504.7.4~1/RELEASE_I386