For added utility, convert the array to a hash:

sub extract_fields {
   my @anotherlist;
   my $ptr = 0;

   while (@list) {
      push(@anotherlist, shift(@list));
      my $length = shift(@list);
      if ($ptr >= length($string)) {
         push(@anotherlist, undef);
      } else {
         push(@anotherlist, substr($string, $ptr, $length));
      }
      $ptr += $length;
   }

   push(@anotherlist, '_leftover', substr($string, $ptr))
      if ($ptr < length($string));

   return @anotherlist;
}

%fields = extract_fields(...);
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