DB<168> $str="abcd"
 
  DB<169> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<170> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
0::
  DB<171> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<172> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<173> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<174> pos($str)=0
 
  DB<175> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
0::
  DB<176> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<177> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<178> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<179> pos($str)=undef
 
  DB<180> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<181> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1:
  DB<182> print pos($str),":",scalar ($str=~/(.)\G/g),":",$1
:1: