Old way in more detail

• • qr/
  • \( # Open paren
  • (?> # Possessive subgroup
  • (?> [^()]+ ) # Grab all the non parens we can
  • | # or
  • (??{$pat}) # Recurse, grab a balanced paren
  • )* # Zero or more times
  • \) # Close paren
  • /x;