>perl -wMstrict -le
"$_ = 'abcdef';
 print_pos('initial');
 ;;
 m{ \b }xmsg;
 print_pos('single \b/g');
 ;;
 m{ \b }xmsg;
 m{ \b }xmsg;
 print_pos('double \b/g');
 ;;
 m{ \b }xmsg;
 m{ \b }xmsg;
 m{ \b }xmsg;
 print_pos('triple \b/g');
 ;;;;
 sub print_pos {
   printf qq{%14s: pos = %s \n},
          $_[0], defined(pos) ? pos() : 'undef'
          ;
   pos = undef;
   }
"
       initial: pos = undef
   single \b/g: pos = 0
   double \b/g: pos = 6
   triple \b/g: pos = undef