sub R {
   printf qq{before: \$_ is '$_'};
   printf qq{  \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined';
   m/(\d+)/g ? $1 + R() : 0;
   printf qq{after: \$_ is '$_'};
   printf qq{ \$1 is %s \n}, defined($1) ? qq{'$1'} : 'undefined';
}
$_ = 'a81b2d34c1';
R();
__END__
before: $_ is 'a81b2d34c1'  $1 is undefined 
before: $_ is 'a81b2d34c1'  $1 is '81' 
before: $_ is 'a81b2d34c1'  $1 is '2' 
before: $_ is 'a81b2d34c1'  $1 is '34' 
before: $_ is 'a81b2d34c1'  $1 is '1' 
after: $_ is 'a81b2d34c1' $1 is '1' 
after: $_ is 'a81b2d34c1' $1 is '1' 
after: $_ is 'a81b2d34c1' $1 is '1' 
after: $_ is 'a81b2d34c1' $1 is '1' 
after: $_ is 'a81b2d34c1' $1 is '1'