$sexp = qr/
    (?<sexp>
        \s* \( \s* (?&sexp)* \s* \) \s*
      | \s* (?<atom>\w+) \s*
    )/x;

'(A (B C))' =~ /$sexp/;

## AFTERWARDS

%/ = (sexp => {
    0 => '(A (B C))',
    sexp => [{ 0 => 'A', atom => 'A' },
             { 0 => '(B C)',
               sexp => [{ 0 => 'B', atom => 'B' },
                        { 0 => 'C', atom => 'C' }] }]};

@/ = ('(A (B C))',
      [['A', 'A'],
       ['(B C)', [['B', 'B'],
                  ['C', 'C']]]]);