For clear definition, as $_ value got changed in every recursive call of R() function, $1 value also got changed and in return of every recursive call, the last changed value available and that is the reason for getting '1' 5 times in $1. For better understanding, here below I have shown how that recursive calls and values of $_ and $1 will be.

Initial ( First ) callback of R() function:
$_='x55x666x7777x1x'; $_=undefined;
Inside code, after evaluating s/(\d+)// ? $1 + R() : 0;
$1=55; #immediately after executing s// command
 Second callback of R():
 $_='xx666x7777x1x'; $1=55;
 Inside code, after evaluating s/(\d+)// ? $1 + R() : 0;
 $1=666; #immediately after executing s// command
  Third callback of R():
  $_='xxx7777x1x'; $1=666;
  Inside code, after evaluating s/(\d+)// ? $1 + R() : 0;
  $1=7777; #immediately after executing s// command
   Fourth callback of R():
   $_='xxxx1x'; $1=7777;
   Inside code, after evaluating s/(\d+)// ? $1 + R() : 0;
   $1=1; #immediately after executing s// command
    Fifth callback of R():
    $_='xxxxx'; $1=1;
    Inside code, after evaluating s/(\d+)// ? $1 + R() : 0;
    # no callback executed as ( there are no digits ) pattern not matched
    $1=undefined; #immediately after executing s// command
    # after prints 'xxxxx' and '1' for fifth callback
   # after prints 'xxxxx' and '1' for fourth callback
  # after prints 'xxxxx' and '1' for third callback
 # after prints 'xxxxx' and '1' for second callback
# after prints 'xxxxx' and '1' for first callback

Kindly regret me for format. I unable to get exact stack of recursive calls for presentation.