use re 'debug';


$np = qr{
           \(
           (?:
              (?> [^()]+ ) # Non–parens without backtracking
           |
              (??{ $np }) # Group with matching parens
           )*
           \)
     }x;

$funpat = qr/\w+$np/;

"fun(1,(2),5)" =~ /^$funpat$/; #Matches!"