use re 'debug';
my $braces = qr/(?<braces>\{ ([^\{\}]* | (?&braces))*? \} )/x;
#                                    ^

Compiling REx "(?<braces>\{ ([^\{\}]* | (?&braces))*? \} )"
Final program:
   1: OPEN1 'braces' (3)
   3:   EXACT <{> (5)
   5:   MINMOD (6)
   6:   CURLYX[0] {0,32767} (30)
   8:     OPEN2 (10)
  10:       BRANCH (23)
  11:         STAR (27)
  12:           ANYOF[\0-z|~-\377][{unicode_all}] (0)
  23:       BRANCH (FAIL)
  24:         GOSUB1[-23] (27)
  27:     CLOSE2 (29)
  29:   WHILEM[2/1] (0)
  30:   NOTHING (31)
  31:   EXACT <}> (33)
  33: CLOSE1 'braces' (35)
  35: END (0)
anchored "{" at 0 floating "}" at 1..2147483647 (checking floating) minlen 2 
Freeing REx: "(?<braces>\{ ([^\{\}]* | (?&braces))*? \} )"