moritz:
Really gotta go now, but just one more experiment: without recursion (actually, along the lines LanX has already posted).

>perl -wMstrict -le
"$_ = 'x77x888x1x';
 ;;
 'zonk' =~ m{(\w+)}xms;
 ;;
 print 'before: $1 is ', defined($1) ? qq{'$1'} : 'undefined';
 print X1();
 print 'after: $1 is ', defined($1) ? qq{'$1'} : 'undefined';
 ;;
 sub X1 { s/(\d+)//;  print qq{  '$1'};  return $1 + X2(); }
 sub X2 { s/(\d+)//;  print qq{  '$1'};  return $1 + X3(); }
 sub X3 { s/(\d+)//;  print qq{  '$1'};  return $1;        }
"
before: $1 is 'zonk'
  '77'
  '888'
  '1'
966
after: $1 is 'zonk'
[download]

Without recursion, value of $1 seems to be scoped to the subroutine 'block'.
No difference in results for  Xn() + $1 versus  $1 + Xn() or for $1 being undefined initially.