s-expression regularexp

use strict;
use warnings;
use Data::Dumper;

my $expr = "(print (add 3 (add (min 9 3) 4)))";

my $stack = [];

sub op {
  push @$stack, [];
}
sub clos {
  my $t = pop @$stack;
  my $n = pop @$stack;
  push @$n, $t;
  push @$stack, $n;
}

sub st {
    my $x = shift;
    my $t = pop @$stack;
    push @$t, {token => $x};
    push @$stack, $t;
}

my $r = qr {
    (?(DEFINE)
        (?<WS> \s*)
        (?<l> \( (?{&op()}) )
        (?<r> \) (?{&clos()}))
        (?<token>(?&WS)
            ([a-zA-Z0-9]+)
            (?&WS) (?{&st($+)}))
        (?<expr> (?&l)(?&WS) (?: (?&expr)+  | (?&token)+ )+ (?&WS) (?&r) )

    )
    (?<tks> (?&expr))

}six;
if ($expr =~ $r) {
    print Dumper($stack);
}