>perl -wMstrict -le
"$_ = 'x55x666x7777x1x';
 ;;
 print 'before: $1 is ', defined($1) ? qq{'$1'} : 'undefined';
 print R();
 print 'after: $1 is ', defined($1) ? qq{'$1'} : 'undefined';
 ;;
 sub R {
   printf qq{  \$_ is '$_'};
   printf qq{  \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined';
   return s/(\d+)// ? $1 + R() : 0;
   }
"
before: $1 is undefined
  $_ is 'x55x666x7777x1x'  $1 is undefined
  $_ is 'xx666x7777x1x'  $1 is '55'
  $_ is 'xxx7777x1x'  $1 is '666'
  $_ is 'xxxx1x'  $1 is '7777'
  $_ is 'xxxxx'  $1 is '1'
4
after: $1 is undefined